I've got several things that I haven't used - yet, but I intend on using them. Blight Magic, Chaos Magic, Alchemy and Herbalists, for example. All will be a part of my campaign some day. Urban Blight, and Bluffside, and all the other stuff from Meg/TG forming a core of it.
Other stuff that I've got that I may never use:
FRCS (yes, it's beautiful and all, but I don't play in FR)
MotP (again, beautiful, but I doubt my PCs will be doing any planar hopping IMC)
I haven't used DotF yet. My TaB is going to see some use when my sorcerer can take the MindBender PrC.
Dragonstar - I guess this isn't really true, because I did play in a PBEM of this. I quit when the DM allowed a Psionic bunny in the game.
I purchased the original RuneQuest boxed set on Ebay a number of years ago. My friends and I were trying to make our own RPG system, and someone commented that some of the things that we were trying to do sounded a bit like RuneQuest to them. So, I picked it up. Not sure I've ever really looked through it, though.
Of course, I've got all that 2nd edition stuff that I'll never use again.