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

[Henrix]

The classical answers is of course to seek out the grave of Roland and get the Horn of Fate, a.k.a. Olifant, in order to end your own world and rebuild it again.

 

[Kzach]

What riches would you liberate from these earth people to take back to your native plane?

Your mom.

 

[Theo R Cwithin]

Credit Cards. After having the nature of credit explained (but not properly understood) my character would take credit cards. Thousands of money on just this little thing? That's better than a portable hole!

segue back to Fantasy World and a Ye Olde Magic Shoppe and trying to purchase the Weapon of Ultimate Arse Spanking.

PC: "It's Visa."
NPC: "Don't look like a viser. How's it attach to the helmet?"

And after PCs get their 'es kicked, the rogue still has a nice tool that gives +2 on open doors checks.

 

[Theo R Cwithin]

Excalibur, the Holy Grail, etc.

Chemistry101 textbook, boomstick, etc.

a copy of D&D. (but WHICH EDITION????)

 

[Dausuul]

Much depends on the setting and edition the characters are coming from. If they're coming from 3E Eberron, their wants will be very different from characters coming from 4E Dark Sun.

That said, you really can't go wrong with nuclear weapons.

 

[DreadPirateMurphy]

A hydrogen bomb. Imagine that explosion amplified by magic.

What could you possibly need to kill that a hydrogen bomb by itself wouldn't?

 

[Kzach]

What could you possibly need to kill that a hydrogen bomb by itself wouldn't?

Bigger is always better, irrespective of fact or logic.

 

[CubeKnight]

What could you possibly need to kill that a hydrogen bomb by itself wouldn't?

Far Realm Creatures
See: Eternal Darkness. Okay, it was not an hydrogen bomb, but it was still a magically-empowered bomb

 

[Bullgrit]

The classic answer would be the powerful artifacts that are lying around in museums that no one in this world can use, but that will work just fine back in fantasy world. Was it Dragon #100 that had the adventure that sent the PCs to London to collect a magic blade?

A mace. The mace of St. Cuthbert, (Greyhawk deity), specifically.

It looked like a fun adventure, but my favorite thing about the module was how you could see The Doctor and the Tardis as a random encounter.

Bullgrit

 

[Mustrum_Ridcully]

Far Realm Creatures
See: Eternal Darkness. Okay, it was not an hydrogen bomb, but it was still a magically-empowered bomb

Great, now it's not a Gibbering Mouther that warps reality, but it's a radioactive Gibbering Mouther that warps reality and shoots gamma ray beams from his numerous eyes, and can cast reality-burning-and-warping fireballs at-will.