You can now snag the Early Access of Smash Up on Steam for free. You’ll have access to the Dinosaurs, Ninjas, and Geeks in the standard version, while you can purchase the Zombies and Aliens separately for $2 each. There’s also a Start Pack for $9 that includes the Zombies and Aliens for now, but will add 4 more factions later once they’re released.
I’m at work right now, so I haven’t had a chance to give this a shot. I’ll make sure I post a bit more once I have a chance to really dig it.