* Either the Complete series or the Races series -- it depends on what appeals to you more. Either series gives just enough options to spice up your game, but not too much for overkill (like it would if you went down both routes). D&D classes tickle my imagination more than its races so I went with the Complete series.
If you DM a lot:
* Dungeon Masters Guide 2 -- A lot of valuable advice and aids to help statting NPC's and towns. Adds some other neat features for magic weapons and little flavor tweaks to keep things interesting.
* The Battlebox and Dungeon Counters from Firey Dragon are a great help. If you're like me and not a minis guy, their counter packs are very nice as well.
If you need ideas for a session:
* The Goodman games dungeon modules are excellent.
If you need ideas for a campaign:
* The Wilderlands Box Set, while pricey, is very good. It doesn't overwhelm you with a lot of setting detail, but there are literally hundreds of little hooks, nooks and crannies to explore. You can painlessly drop any of the Goodman games modules in there with almost no effort. If fact, I plan on doing just that