Mage:The Ascension, I love the setting, Love the magis system , I'm ok with storyteller rules in general (and I love the streamlined exalted variant), but I could never find someone who wanted to play. Boo !
3G3 ...I bought it quite a while ago with the intention of using it to do firearm rules for my homebrew. The math scared me away a bit, and I just never got back around to getting into it.
oh, and Call of Cthulhu (20th anniversery edition). I picked it up because the book looed so cool, but I've yet to find the time to read it let alone run it.
Buffy RPG (I'm scared of doing it wrong*) and Call of Cthulu d20. Games I bought & I'm not sure I want to run include Slaine d20 and Traveller d20.
*I don't think I've ever felt this way about any other game. I'm planning to start a Conan game soon and I have no worries about mucking it up. The Buffy RPG is a beautifully crafted work of art but it has a really heavy emphasis on the TV-show format. The Drama Points seem much more central to task resolution, almost oppressively so, but I can't get rid of them without probably turning it into Buffy-Cthulu (everyone dies).
I love my Kalamar Atlas... but I just can't get psyched about the setting, especially now that I have my low and high magic needs catered for by Midnight and Dawnforge, respectively.
I just wish it were economical to produce the same sort of atlas for the worlds I actually DM in!