Alternity: Own pretty much all of it, never got the game off the ground. I'm hoping to get one together once D20 Modern comes out, but I'm having way to much fun playing D&D right now.
Psionics Handbook: Cool ideas, poor implementation and I don't have the time to make it work.
Lots of adventures: I collect adventures but don't have the time to run them all and I like to write my own. So, I end up with lots of never run adventures sitting on the shelf, just waiting for the day I'll pick them up and run them.
Wizard Spell Compendiums: Bought them and then 3E came out. I didn't feel like adding them into the campaign until I got a good sense of how the spell balanced out. I might start adding some in fairly soon though.
Sovern Stone Codex Mysteria: I love the idea of how magic works, but my players are more interested in playing traditional 3E casters.