Purchases I regret:
Warcraft RPG
Gamma World d20 (brutally disappointing)
Forgotten Realms Campaign Setting, and supplements
Masters of Arms
Song and Silence
Masters of the Wild (the other three splats are fine by me)
Spycraft and Shadowforce Archer
Star Wars (both editions) and supplements
Keepers:
The 3.0 and 3.5 core books (though, I'd be glad to trade those 3.0 books in)
d20 Modern (best implementation of d20, bar none)
d20 Modern Menace Manual
Urban Arcana (much as I hate the setting, it's perfect for converting Modern to pseudo-medieval fantasy)
Modern Player's Companion
XCrawl
Dragonstar and supplements
Oriental Adventures
My Dungeon and Dragon subscriptions
Four Color to Fantasy (the PDF as far as I am concerned. I'd like to see a print version)
Mindscapes (I'm not using the psionic combat, but the class revisions are a big help)
edit: Savage Species (how did I miss this?)
I'm actually surprised at how much ended up on the first list-- I'm generally good at not buying things I won't use. A lot of the FR stuff can be blamed on playing with people who used to be hell-bent for leather FR fanatics-- now that they don't normally play in the FR, I'm staring at these books and wondering why they're not catching fire.
Gamma World is a bitter, bitter disappointment. It's thirty-five dollars and three hours of my life I will never get back-- compounded with the fact that older editions of Gamma World were my favorite RPG, and it's actually impossible to play the kind of Gamma World games I enjoyed with these new rules. I'm going to go back to my original plan of designing my own conversion, probably using an adaptation of the 4C2F rules for mutations.
Star Wars is, to me, like Dragonlance. It's an incredible setting to read about, and it has some awe-inspiring sagas that will make you want to play it... but then you realize that your characters are either stuck permanently in the shadows of the original heroes, are forced to play things exactly as the original heroes did, or are simply completely and utterly irrelevant to the game world as a whole. Not very appealing.
Out of my regretted purchases list, I think Spycraft is the best-designed game, but I just prefer d20 Modern.