rRootage is a smart - and free - Gridrunner-esque vertical-scrolling shoot-em-up for OS X. There's a certain mathematical feel to the hailstorm of enemy fire, like fighting inside Geiss, and the bad-guy spaceships look like they're generated with a Biomorphs algorithm. I get the feeling it would be interesting to fight a scrolling crop of algorithmically-bred mutants that evolved around their weaknesses to your attacks.