Here's another idea I had mocked up a couple of weekends ago, but in the interim I had my laptop taken to the drycleaners to get fixed up, and when I got back I couldn't get SDL to work anymore, so I ended up doing it again in allegro. Anyway, it's a simple enough concept-game for two players. One player is a heat-source, the other player can create waves by pressing spacebar. I figured that it might, in theory, be possible to create some maps such that it might be fun for them to go head-to-head.
I'm not so sure now if it's possible. I haven't actually tried playing this multiplayer though. Possibly with some tweaking it might work out. (for instance, the heat player can totally toast up enclosed spaces, whereas the wave dude can get a lot more range by splashing while standing against a wall). I don't think there's much point in trying to do any fine-tuning or adding any extra features until I've played it multiplayer myself
Heatwave for Mac OS X (20KB)
Heatwave for Windows (264KB)
Controls:
Player 1: Cursor Keys and Spacebar
Player 2: W S A D