((Ok, a long time ago, I tried D&D, and I've decided that I'll never touch that particular stuff again for various reasons that I'd rather not get into, but I feel that I can help you out, because I actually found reading through supplement books to be more fun that actually playing the game, mostly because I didn't have to deal with people to do so, and if you mix multiple supplement books, you can get some unbelievable results.))
The first book I would recommend is the 2nd player's handbook, although it doesn't add any particularly wacky things, it's nice and gives plenty of cool features to all of the original classes to make them more customizable. It lets rouges throw spells back at wizards, and monks use hadoken.
The second book I'd suggest is the Eborron (I know I misspelled it) Book, although the Warforged are pretty dumb, in my opinion, the Factotum class and some of the feats (particularly the Dragonmark ones) are pretty cool.
The various books involving new races (Races of the Wilds, Races of Destiny, etc.) are some of my favorites. My personal favorite race are the Illumians, which are specifically designed with multi-classing in mind. While Humans and Half-elves get favorite class "any", they might as well get favorite class "all" in comparison. (I'm a sucker for multi-classing)
Other than those, the many "Complete" books are pretty good (Complete Warrior, Complete Mage, etc.) for when you're tired of the base classes.
As for Psionics, I personally thought it was kind of stupid, since it was basically magic with a mana bar, rather than spells per day.
Except for the Soulknife, they were ok.