Just a list of Cowboy games
Boot Hill - TSR ('75 & '79 editions very similar, '90 edition is reputedly very different mechanically)
Gunslinger - AH wargame
Wild West - FGU, 1981 had a location supplement called Widow's Peak or somesuch
Western Hero/Outlaw (essentially the same supplement statted for HERO and Rolemaster)
GURPS Old West
Aces & Eights - Kenzer & Co.
Sidewinder/Sidewinder Recoiled - d20, IIRC
generic Wild West supplements: Knuckleduster Firearms Shop and Cowtown Creator
PS - Knuckleduster.com friggin' rules!
New Book - It folds, so you can page through it or spread it out as needed.