Boot Hill was a wild west roleplaying game published by TSR in 1975. The second edition was published 1979, and then the third edition was published in 1990 (that last date may be wrong, I have yet to find an actual copy to look at, but it was released sometime AFTER that date.) The game was originally created by Gary Gygax and Brian Blume, after TSR had already published Dungeons and Dragons.
The game was based on a five attributes: Strength, Coordination, Observation, Stature, and Luck. It did not have any sort of stat for alignment, which was a break from Dungeons and Dragons. Everything in the game was based on a percentile system to determine success, as opposed to the polyhedral dice collection that D&D would require.
Boot Hill (along with Top Secret, Star Frontiers, Metamorphosis Alpha, and Gamma World) may have been responsible for some of the push of modern games to make universal roleplaying games. (See GURPS, Fuzion, Hero System, and Fudge.) In the back of the old first edition AD&D Dungeon Master's Guide there were a few pages (112-114) which dealt with integrating Boot Hill characters into your D&D game, and vice versa. The early pioneers to roleplaying weren't much different from what you might see in a home campaign. They wanted to have Merlin meet Wyatt Earp, Flash Gordon fight a Dragon, and Billy the Kid deal with zombies. (e.g. the spaceship in Expedition to the Barrier Peaks and the fact that Murlynd from Greyhawk carries around six shooters.)
One of the problems that Boot Hill ran into was the "incompatible edition" problem that many roleplaying games run into. Boot Hill dealt very poorly with it, and attempted no sort of compatibility between first and second, or second and third editions. When you do this you risk alienating your hard core fans, which in many cases are your best advertisements. (Wizards of the Coast was very concerned with this with the creation of Dungeons and Dragons Third Edition, and you can see where they avoided changing some sacred cows in order to placate the old hard core gamers.) People also distrusted the fact that it made no attempt to be compatible with Dungeons and Dragons, not even on a shallow level.
Another problem is that Boot Hill didn't have a setting to capture the player's minds. Boot Hill advertised itself as a Wild West game, but didn't try and get you excited about the prospect of playing a Wild West game. You were expected to play it if, and only if, you liked that setting. This is in contrast to a newer entry into the Wild West, Deadlands, which lures you in with a hook. "This isn't the wild west. This is the WEIRD wild west." I, personally, would not have ever wanted to play Boot Hill (apart from to learn the mechanics of the system, due to the chart in the back of the DMG), but Deadlands still seems attractive to me.
Boot Hill, however, was really a predecessor of what roleplaying games would be. Dungeons and Dragons proved the concept of roleplaying games in general. Boot Hill demonstrated that you could have a game with a touch of a generic setting. With a few tweaks of Boot Hill, you could have modern fire arms, vehicles, and play Top Secret using just Boot Hill rules. Or change a few other numbers, make the pistols into muskets and play in the American revolutionary war.
Apart from living on in the hearts of some old school gamers (see all the Cattlepunk references in Knights of the Dinner Table), Boot Hill will most likely remain out of print for some time.