Some of the things I would
not have bought if I would have put some thought into it.
1. Unfortunately most of my Mongoose purchases. Especially the
Quintessential Books. They have been a waste for me and have seen no use at all.
Their B5 line, I only have the first 3 books but no one wants to play.
I like their
Slayers Guide but once again I just haven't been able to use them.
I'm interested in their
Classic series but at $34.95 a book... well I can't get everything.
Though a few of their
Encyclopedia Arcane books have seen some use.
2. Anything by Fast Forward Entertainment, complete disappointment- rules, editing, art, ect.; though I do find some of the 'story' ideas interesting.
The product of theirs that I'm interested in the most is one that has been delayed for well over a year...
1492: Conquista del Magico.
3. Avalanche Press, I like their historical books but I once again I just haven't used them.
4. WotC
Star Wars line, see my B5 reference above.
5. I was excited when I got the splat books but no one in my group has yet to take a PrC, even the one that I made.

Though some of the feats and spells have been used.
6.
Hero Builders, enough said.
Things I've used and happy I got...
Core books- 3.0 & 3.5 (wouldn't have a D&D game without them)
I use Psionics so the PsiHB, the expanded PsiHB is on my 'must buy' list.
NG, GR, Malhovac (though I have AU I haven't used it yet), and Bad Axe Games (best books for the price I've seen yet

) products.
My goal this year is debt reduction so I won't be buying much this year. I only have 3 left on my 'must buy' list:
Book of Fiends (GR),
ExPsiHB (WotC), and now NG's
Tome of Horrors II, I just got
Fields of Blood (Eden). Anything else I'll have to justify to myself (and others now that my life has gotten more complacated for the better) to purchase anything over $20.