A portal takes you to a place called 'Earth'. What do you loot?

[Man in the Funny Hat]

Binoculars? Think bigger - laser sights, nightvision gear, infrared scopes, for that matter - a few good Predator drones.
Communication is always at a premium in D&D - grab some good tactical communications gear as well as long-range communications gear.
Cases of nice, white, heavy-bond paper whether or not an actual photocopier will work.
Linotype and old belt-driven printing press? How about just a hand-cranked mimeograph machine?
A nice high-end laptop and Campaign Cartographer 3
M1 Abrams and/or Apache and/or A-10 Thunderbolt. Just one of each should do.
A phased plasma rifle in the 40-watt range?
"Hey, just what you see pal..."
Any number of crates of M-16a4's with scopes, lasers, night vision, etc. - ammo too, of course.
Oh, and who needs pron when there's enough whores available for every orifice and extremity you might have - and cure disease?

Another question would be what do you grab to take back from D&D world to Earth? My only suggestion: gold. Lots of gold. Okay, and diamonds.

 

[chronoplasm]

You're making the assumption that, after seeing this incredibly magical paradise of modern Earth, that adventurers would want to go back.

I'd probably just stay.

But what about adventure? There aren't as many dungeon crawls on earth.
Except for "The Long Stairs" of course, but that's a closely held government secret.

 

[jdrakeh]

But what about adventure? There aren't as many dungeon crawls on earth.

True. Here you can earn enough money to buy a home without risking your life. Who would be interested in that?

 

[Man in the Funny Hat]

That would probably be the most valuable thing you could bring back, but I think it would be very hard. I'm inclined to think that the viability of modern representative democracy as a political system is significantly dependent on technology.

ARTHUR: Old woman!
DENNIS: Man!
ARTHUR: Old Man, sorry. What knight lives in that castle over there?
DENNIS: I'm thirty seven.
ARTHUR: What?
DENNIS: I'm thirty seven. I'm not old!
ARTHUR: Well, I can't just call you `Man'.
DENNIS: Well, you could say `Dennis'.
ARTHUR: Well, I didn't know you were called `Dennis.'
DENNIS: Well, you didn't bother to find out, did you?
ARTHUR: I did say sorry about the `old woman,' but from the behind you looked...
DENNIS: What I object to is you automatically treat me like an inferior!
ARTHUR: Well, I AM king...
DENNIS: Oh king, eh, very nice. An' how'd you get that, eh? By exploitin' the workers; by 'angin' on to outdated imperialist dogma which perpetuates the economic an' social differences in our society! If there's ever going to be any progress...
WOMAN: Dennis, there's some lovely filth down here. Oh! How d'you do?
ARTHUR: How do you do, good lady? I am Arthur, King of the Britons. Who's castle is that?
WOMAN: King of the who?
ARTHUR: The Britons.
WOMAN: Who are the Britons?
ARTHUR: Well, we all are. we're all Britons and I am your king.
WOMAN: I didn't know we had a king. I thought we were an autonomous collective.
DENNIS: You're fooling yourself. We're living in a dictatorship. A self-perpetuating autocracy in which the working classes...
WOMAN: Oh there you go, bringing class into it again.
DENNIS: That's what it's all about if only people would...
ARTHUR: Please, please good people. I am in haste. Who lives in that castle?
WOMAN: No one lives there.
ARTHUR: Then who is your lord?
WOMAN: We don't have a lord.
ARTHUR: What?
DENNIS: I told you. We're an anarcho-syndicalist commune. We take it in turns to act as a sort of executive officer for the week.
ARTHUR: Yes.
DENNIS: But all the decisions of that officer have to be ratified at a special biweekly meeting...
ARTHUR: Yes, I see.
DENNIS: By a simple majority in the case of purely internal affairs...
ARTHUR: Be quiet!
DENNIS: ...but by a two-thirds majority in the case of more...
ARTHUR: Be quiet! I order you to be quiet!
WOMAN: Order, eh? Who does he think he is?
ARTHUR: I am your king!
WOMAN: Well, I didn't vote for you.
ARTHUR: You don't vote for kings.
WOMAN: Well, 'ow did you become king then?
ARTHUR: The Lady of the Lake,
[angels sing]
her arm clad in the purest shimmering samite, held aloft Excalibur from the bosom of the water signifying by Divine Providence that I, Arthur, was to carry Excalibur.
[singing stops]
That is why I am your king!
DENNIS: Listen, strange women lying in ponds distributing swords is no basis for a system of government. Supreme executive power derives from a mandate from the masses, not from some farcical aquatic ceremony.
ARTHUR: Be quiet!
DENNIS: Well you can't expect to wield supreme executive power just 'cause some watery tart threw a sword at you!
ARTHUR: Shut up!
DENNIS: I mean, if I went around sayin' I was an empereror just because some moistened bint had lobbed a scimitar at me they'd put me away!
ARTHUR: Shut up! Will you shut up!
DENNIS: Ah, now we see the violence inherent in the system.
ARTHUR: Shut up!
DENNIS: Oh! Come and see the violence inherent in the system! HELP! HELP! I'm being repressed!
ARTHUR: Bloody peasant!
DENNIS: Oh, what a give away. Did you hear that? Did you hear that, eh? That's what I'm on about. Did you see him repressing me? You saw it didn't you?

 

[Celebrim]

Some assumptions:

a) I'm from some place magical.
b) I'm from some place that thinks a crossbow is high tech.
c) That place is otherwise Earth (Earth foilage, 24 hour days, inhabited by humans, 1g gravity, etc.)

Some goals:

a) Bring back something superior to what exists on my world.
b) Bring back something that I can be understood by local craftsman and repaired when it inevitably breaks.
c) Bring back something that will make me fabulously wealthy, and hense powerful.
d) Bring back something that I'm sure will work even when copied and whose function won't be tied to 'magical' materials from the other world.

#1) Modern timepeices, and in particular a couple brass wind-up ship's clocks (such as a Harrison timepeice). This will allow calculation of longitude, which will be invaluable to sailors without powerful divination spells. A couple extra ones will be invaluable because the basic clockwork technology shouldn't be beyond my world's best metalurgists, which means that by taking a couple apart I should be able to duplicate them and thus begin a very profitable venture.
#2) Telescopes/Binoculars: These peices of invaluable military technology generally don't exist in fantasy worlds, but the basic glass grinding technology usually does exist in a sufficiently advanced state that once a few lens are shown to glass grinders in my employ they will be able to understand the basic principles involved in their manufacture. While it's less certain that optics work in a magical world (light might pass through the native curved glass of different thicknesses without bending) than it is that clockwork functions, there is a good chance deflection and diffraction and the like all work in my fantasy universe.
#3) A Jaquard Loom: Like my clockwork timepeice, there is nothing about this extraordinary invention that can't be duplicated by a sufficiently good fantasy world craftsman that is allowed to inspect and investigate it and likewise the device only depends on the sort of physics that I can be confident actually work in my world. Possession of a Jaquard Loom will revolutionize textile production, allowing me to essentially take over the world wide market for cloth. The insights that the object will provide in the manufacture of machines and clockwork calculating devices may well usher in a 'clockpunk' age on my world.
#4) A 1924 Doble Model E Steam Car: This car can operate using pretty much any liquid fuel - 'flaming oil' is a pretty good candiate - and achieve speeds on flat ground in excess of 60mph (which is a base move of around 20 squares). The construction of the vehical is simple compared to any modern automobile (no electronics, no transmission, no differential gears, etc.) and involves no technology that couldn't be understood and repaired after investigation by a decent blacksmith. The compact steam engine and basic design could become the prototype from which various sorts of steam powered devices could be made. Of course, it's entirely possible that the gas laws upon which the car's function depend don't work in my fantasy world (hot air might not exhert a particularly large pressure), but its a reasonable risk.
#5) A modern mountain bicycle: The aluminum in this vehical probably couldn't be duplicated easily and the rubber might be tricky as well, but its possible that a suitable fantasy substitute for a lightweight strong metal could be found. In any event, the basic design if it could be manufactured would revolutionize long distance communication in my world even if the mass produced version was slightly heavier and more cumbersome. Provided you had a basic road, messages could be sent quickly without the need for a horse and large parties could travel great distances without the need for foraging for the horses. I envision something like a Roman legion mounted on bicycles. Instead of astounding the world by being able to move my troops 20 miles in a day, on roads of the quality of Roman ones I could move a large formation 100 miles - equaling the strategic mobility of Greek galleys but doing it overland.
#6) A modern compound bow: A modern hunting bow would astound the archers of the ancient world with the ease it could be drawn and the accuracy even a relatively untrained archer could manage. While the materials would be difficult or impossible to duplicate, it wouldn't be that hard for a skilled bowyer to come up with suitible substitudes in wood, horn, and metal and incorporate the ideas (pulleys!) into a fantasy version that had all the accuracy and power of far more expensive magical weapons.

 

[El Mahdi]

But what about adventure? There aren't as many dungeon crawls on earth. ...

Yeah, but there are Pub Crawls!

And modern dance clubs...forget about it. Lasers, fog machines, colored spot lights, and Disco Balls would beat out even the best Illusion Spell or Dancing Lights.

Of course though, the metal detectors at a lot of clubs nowadays would probably pose quite a problem for an adventuring group.

 

[Mark]

Three things. A set of encyclopedias, an ice cream maker and a nerd.

 

[Dioltach]

A portable DVD player, a large-screen TV and the Firefly DVDs.

 

[Mustrum_Ridcully]

Books. Or a computer with a lot of books and a copy of Wikipedia and any other online archive there might be. The computer thing might be tough to grasp for a fantasy hero (or not, it's just some kind of magic). What you need is the ability to get all the advancements of our world to the fantasy world and recreate them there. This means you need all there is to know about it.

It doesn't even have to be about weapons or cars. Knowledge about biology and agriculture can be useful enough already. Knowing about the theories of evolution or just Mendel's genetics could help peasants a lot to create more sustainable crops and breed better farm animals. If no one takes you on it, just build your own farm and sell the seed, your knowledge will be put to good use regardless of whether people understand it or not.

If the wizard can recharge it, an electric car would probably be cool to take with you. Of course, this doesn't solve the issue of maintenance. Maybe a Make Whole spell will do that just fine though.

But it might be great to also know how to produce electricity on your own, so a generator and maybe the blueprints for a generator (and the blueprints for the tools required to create them, and the blueprints for the tools required to create the tools?) would be cool.

Weapons are always welcome to our Hack & Slash heroes. Any Rifle or Pistol will prove useful, but you also have to know how to create the bullets and maintain the weapon. The weapons might even be enchantable later. After all, if all the magic of the party still works on Earth, it stands to reason it will also help enchanting stuff.

 

[Mustrum_Ridcully]

Three things. A set of encyclopedias, an ice cream maker and a nerd.

I think encyclopedias - even the digital, big ones - might not contain enough information.

If there was ever a need for a reality show, then this is it:
"Wiki People"
Concept: 24 People are stranded on a lone island. The start with nothing but
- Their Clothes
- Food and Water for 30 days
- Basic Tools (hammer, saws, whetstones and similar things)
- Access to Wikipedia (all languages)
- A plan on what to build or grow with a suggested order

How long will it take them to reach the first items on their list? Will they manage, or utterly fail?

Or is this already the concept of Survivor?