L.A.W (Living After War). Sure there's Fallout, Wasteland, and when it comes to MMOs, Fallen Earth, but the more the merrier.
Luckily for us (and you), L.A.W is gearing up for its first closed beta phase, and the title is available for download at its official website. You'll need a beta key to participate, of course, and if you don't have one yet, you'll need to sign up for an alaplaya account.
L.A.W is described by alaplaya as a PvP-centric MMOG with an RTS mode, and it "also features classic role-playing elements such as a guild system, dungeons, and extensive skill sets."
[Source: alaplaya press release]