Deadlands (first edition). I loved the setting, and liked the mechanics and gimmicks okay (though the designer's understanding of probability was weak). Just never got around to playing it.
Other RPGs on my bookshelves that I never played (unless noted otherwise, I liked these on reading):
d20 Call of Cthulhu, d20 Future, Digital Burn (early d20 cyberpunk), Arcana Evolved, Iron Heroes, Noir (very rough system, but I'm a sucker for noir), d6 (Fantasy, Space, and Adventure), Boot Hill (not a good system), Savage Worlds (I just finished reading it, and I'm not sure if I like it or not; it's pretty clear there's (minor) wonkiness in the probability of those mechanics, too, though I haven't crunched the numbers).
RPGs on my shelves I wish I got to play more often:
WEG Star Wars, Shadowrun, and GURPS (which just illustrates the masochist in me), and Amber (ditto).