This one is going to be somewhat shorter than my last two posts
Someone finally brought Boot Hill to our little group about 1983 and we played it a lot. I ended up with this cover (below) but the contents were the same
This was a fun if limited game focused mainly on shootouts. You could do whatever you wanted but most of the rules centered around gunfights. We quickly grew tired of this and used the conversion rules in the DM's Guide to mix it with D&D as that was a LOT more exciting. I spent more time in the Lost Cavenrs of Tsojcanth with a group of bank robbers running from the law than I ever did with a regular D&D party. Hostile elves (er, "Funny Lookin' Indians)? Gatling gun. Angry hill giants? Dynamite. It's fun and I recommend it to everyone at least once.
That was the majority of my cowboy gaming for a long time. There have been a few others over the years - GURPS Old West, Western Hero were both nice treatments of the historical aspects but didn't add much. Aces and Eights is a really nice game that actually lets you run a real western campaign without mixing in D&D with rules for ranching and cattle drives and a cool combat system but I've never been able to talk anyone into playing it. It's unfortunate but true as it's a really well done game.
There's only one other Western game worth mentioning in my opinion:
One of my favorite games that I've never played and only run a bit is Deadlands. It's everything cool about westerns + some D&D types stuff + alternate history + interesting mechanical systems - a home run on every level. There was a ton of supplementary material released for it from short story collections to a miniatures game to adventures and regional supplements. The whole thing is great and while it's not a truly historical or realistic old west game (there are mad scientists and magic-users) it is the coolest one by a mile.
If you don't like the original, occasionally clunky system then you are in luck as there is a GURPS conversion. a d20 version, and a Savage Worlds version. I'm a big fan of the original system but I would probably run the Savage Worlds version nowadays - it sacrifices some detail but it works really well. I thought the d20 version was fine but lost a little flavor and the GURPS version is...well...GURPS. It's the same concept expressed in a very different mechanical way and one that might lean a little too far towards "realism" to truly reflect the atmosphere of the original. If you can find a way to work in the cards and poker chips of the original into your system of choice then you are that much closer to capturing it.
As much as I love it I've only run it a few times and all of those were one-off's. Even in Texas (or maybe because we live in Texas*) no one I know has ever wanted to play a long-term western game as the primary or even a secondary game. It's a hard sell it seems and I don't know what it would take to change it if the coolness of this game can't break through the barrier.
I know there are some other western games out there but these are the one's I've played and love so I can't say much about the rest. If you think there's one that's contributed to the genre then let me know in the comments.
*yes I own 4 pairs of boots, no I don't own a hat or a duster (come close a few times though) and no I don't own any cows or oil wells - just to get that straight.