Most indispensible DMing tools.

Check out the 3.5 Resources at the HeroForge site. The HeroForge programme itself makes generating NPC's almost as easy as using the Henry/S'mon short-cut method. MonsterForge is also a good resource: print-out by-the-book nasties, or upgrade their hit dice, skills, abilities, etc., add templates (Fiendish Dire Badger, anyone? ) or powers to the various critters, and all the maths is automatically worked out for you.

Finest set of prep-work resources I have found for 3.5E.

There's also generators for Towns, Castles, Animal Companions, and other odd resources.

Yes - Alea Tools has imprved their marker line to overcome some of the exuberance of their initial magnetic markers - now they function just as well without the occassional problem of too powerful magnets hindering each other in close quarters. The markers are super simple and extreemly versitile - work great alone, with tokens or minatures. Great tool for gaming. Alea Tools has original product on sale and are giving away conversion circles this week with eveyr purchase (GM Day special) - check them out at aleatools.com.

The four main tools I use and probably would die without...

SORD: This saves so much time
Digital SRD: Turning pages is so yesterday, just search and find
NPC Designer: Saves me countless hours during prep and allows me to wing it when things don't go the way I thought it would.
OneNote: I really don't think there is a better tool for managing campaigns.

I'm kinda feeling a bit old-school now, but here's my bit.

I use a very cut-and-paste method to construct the details of my campaign. The most important item I keep is a 13-pocket file with a variety of papers. I find it VERY EASY to build anything from a one-shot adventure, to a full campaign, with a little help from my file folder.

A section devoted to city/town maps.
A section devoted to wilderness maps.

A section devoted to friendly NPCs.
A section devoted to potential villain NPCs.

A section devoted to people pics
A section devoted to monster/beast pics

A section devoted to treasures.

A section devoted to lists of names. This is extremely important for me, because I always have a hard time with names.

A "10-page bound sheet protectors" (find it at office stores), that I keep house rules, and the quick-reference sheets made for D&D 3.5.

Another 10-page bound sheet protectors where I keep current "DM info."
Another 10-page bound sheet protectors where I keep current "Player info."

In the first pocket, I'll keep everything that is currently, "in play" and simply pull it out at the start of the session. These always include the 3 10-page bound sheet protectors, and any pics.

Pictures are full-sized color pages whenever possible, and I clip the 3 newest pics to the DM screen to show off to players, and older pics get moved into the "player's info" bound pages. Any descriptions are kept in the DM's notes.

For my campaign, I always have the "big picture" in my mind, and use my papers to help "fill in the blanks." It ALWAYS makes for saving time and energy when prepping the game.

I don't have to think too hard when I'm collecting all these papers. If I think it looks cool, I grab it, print it, and file it. Just knowing I have these different things in my file is enough to let cool ideas "cook" in my head.

Believe it or not, sometimes it's tough not to use too many ideas at once.