1) A decent Laser printer.
2) A roll of Con-Tac brand clear non-glare (Home Depot, $8)
3) An inexpensive paper-cutter (WalMart, $15)
4) Good scissors (Fiskars are my preferred; WalMart, $15)
5) Ream of cardstock, Letter size (WalMart, $6)
6) Ream of Legal Paper (WalMart, $6)
7) Ream of Letter paper (WalMart, $4)
8) Package of overhead transparencies for said laser printer.
9) A good word processor program (Pages or Papyrus)
10) Mapping and/or CAD programs.
Now for why...
The printer allows printing all kinds of neat handouts; laser printers tend to handle double siding stuff better than inkjets, and have FAR less smudge factor on the output.
The Con-Tac is for making your geomorphs and terrain bits, and your cardboard standups, much more survivable, and for laminating your maps from your various boxed or shink-wrapped sets. Better still, it can be written on in pencil, and a vinyl eraser will nicely lift it.
Cardstock: printing your various maps, paper figures, etc. Laminate it, both sides, and then cut out, and you get YEARS of repeat use.
Legal: if I'm printing a PDF of letter sized pages, I'll often print it, using Acrobat Booklet Printing, on Legal instead of Letter. This gives nicer margins, and places to annotate it. (It also allows a slightly larger printout. Not a whole lot, but just a touch.) Also, if the stock character sheet is too cluttered on letter, a switch to legal allows an extra 3"...
Letter: for normal printing.
Scissors and Paper Cutter. The slider type paper cutters make cutting out your room tiles so much easier. Still, you'll need scissors for some cuts, and a GOOD pair last a LONG time. Don't steal the SigO's sewing shears, tho... Paper and Fabric wear the blades differently, and make them less suitable for the other...
Transparencies: your best friend when making templates... Print to them, then laminate them. THEN cut them out. They now are thick enough to handle easily. Colored transparencies make great templates.
Word Processor: Pages or Papyrus, or if you know it well, Word. All three can produce very nice character sheets, cheat sheets, and screens.
If you have the money and hard drive space, a good mapping program (CC3, Autorealm, Universe, Hexographer) or Cad Program (CC3, Fractal Mapper, Sketchup, Cadintosh, Autocad) are wonderful tools for game aid creation, especially your town maps and castle maps. I use sketchup for making both walk-through views and deckplans for Traveller; deckplans by using the cutaway tools.
Anything more is gravy, but those are my key tech-tools.
In the Gravy list:
An eInk based ebook reader (Sony PRS, Amazon Kindle, B&N Nook (but only the B&W ones), iRex Illiad. For PDF versions. Why waste paper for the lesser used supplements? I prefer to use this over the laptop for reading; I'm one of those who does get screen eyestrain from the LCD.
Laptop with Video Projector or HDTV adaptor. For showing stuff onscreen.
If using Minis and 3D terrain, a laser pointer. For checking LOS easily.