Dungeons and Dragons, Talislanta, Star Frontiers, Star Wars (WEGd6), Torg, StarAce, Chill, Dark Conspiracy, Kult, Macho Women With Guns...
Okay, easy question, hard to answer.
I read all of the games I own, they are a constant source of inspiration for me. Sometimes I will use them as source material for an adventure/session, or at least for the basic plot.
Some of my craziest ideas come from mixing games together. For example, Keep on the Borderlands from D&D becomes a BPN in SLA Industries to remove gangers from an area of Downtown. The Keep itself is the local SHIVER waystation. The same module can be used in Talislanta. The Keep is one of the outposts in the Wilderlands and you are removing a Raknid infestation.
For me, a RPG book has never been a game, it has been a supplement for whatever game I am running.