Old West RPGs
These are Old West RPGs, both self-contained and as add-ons to generic systems.
[A Work in Progress]
Boot Hill. TSR. 1975.
Boot Hill 2nd edition. TSR. 1979, 1984.
Boot Hill 3rd edition. TSR. 1990.
Deadlands. Pinnacle Entertainment Group. 1996.
Deadlands: d20. Pinnacle Entertainment Group. 2001
GURPS Deadlands: Weird West. Steve Jackson Games. 2001
Deadlands: Reloaded. Pinnacle Entertainment Group. 2006.
Sidewinder: Wild West Adventure. Citizen Games. 2002. [d20 system]
Sidewinder: Recoiled. Dog House Rules. 2004 [d20 Modern system]
Dogs in the Vineyard. lumpley games. 2004.
Dust Devils. Chimera Creative. 2002.
Dust Devils Revenged. Chimera Creative. 2007.
Coyote Trail. Precis Intermedia.
Wild West. 1983. Fantasy Games Unlimited.

Aces and Eights.
Gunslingers and Gamblers.
d20 Modern’s d20 Past had some resources.
Werewolf: the Wild West.
And I’m sure I’m forgetting one.
Yeah, I have all those in line to add. I’ve just been remiss, as I want to add links where I can.
There’s actually a great list here:
http://www.erichotz.com/game_rules1.html#rpgames
True West. James L. Shipman.
Go Fer Yer Gun! Roleplaying in the Wild West. Beyond Belief Games.