Nuclear Sunset: The Southwest, is a sandbox style campaign setting for Mutant Future. Mutant Future is the OSR movements re-imaging of Gamma World and other Old School post apocalypse games using the OGL as it's core. There is a link to a free download of Mutant Future on the Free RPG List on the left. Do yourself a favor and grab a copy. Don't worry. I'll wait.
OK, now that you have your rules, we can discuss the setting and what a setting it is. Think Post Apocalypse with classic western overtones, then dial it up.
The combination works better then I would have expected, but then the Firefly TV series showed that sci-fi with strong western overtones can work very well. I give Vigilance Press major props for finding a strong setting concept for Mutant Future that isn't an obvious one, until you read it. It's that good.
Salt Lake, Vegas, Phoenix and other well known locations make the transition to Mutant Future with enough highlights that you'll recognize them, but so much changed that they are totally new.
We are given location write ups, the major factions (and their relation to the different locals), but even more importantly an abundance of adventure hooks. You could easily run a campaign for years using the Nuclear Sunset setting and the adventure hooks supplied. That's a true compliment for a product that is 20 1/2 pages long.
My one complaint is that it is not printer friendly. The nuclear watermark and background art on the text pages will kill your ink. It's not a huge complaint, as I don't plan on printing out a copy (I love my tablets for PDF reading), but I know some of you do like to print your own so I felt this should be pointed out.
Nuclear Sunset is an excellent value at 99 cents. If it's half as much fun to run as it is to read, it's one of the bigger RPG values available.