I’m a big fan of an obscure setting or roleplaying rules set.
I picked up a copy of Qin the Warring States when it came out. The setting is very cool and the rules are outstanding, but I’ve never found anyone to play a game of it.
There’s a yin/yang aspect to the die rolls with one taken from the other, and balance being favourable.
It influenced my DiceBenedict rules if I remember right (I linked to it the other day but didn’t think to more than scan the rules… Yeah, it’s been a long time).
I have a copy of the Smallville roleplaying game because the session zero / game creation rules were recommended and I found it going cheap.
It’s not the greatest game, but it gave me a better idea of how the Cortex rules worked, and the session zero stuff really does a good job of tying the players and setting together with collaborative construction (which is one of the reasons I like FATE).
I have a copy of the Ghostwalk setting for D&D 3e as well. I really liked the book, but at the time of picking it up I didn’t realise it was a stand alone setting.
I’ve used it as an add-on for Cambria in the past, though I’ve never actually run that part of the game (such high hopes that I could make a setting last more than a session sometimes).
It’s really a great setting, and I’d love to throw Manifest into a Planescape game or something to really mess with player’s heads – “A ghost takes corporeal form in front of you and says, in a thin reedy voice, “Hi, do you want to buy a watch?” before opening his spectral coat to reveal rows of very real watches.
(I’d also love to throw a few Wraith: the Oblivion references in as well.)