I picked up Altitude during the free weekend promo, just to try out the multiplayer elements. This and a few other games are among the first multiplayer games in Valve’s Steam for Mac game platform. I highly recommend it! It’s a lot like Subspace, though it supports fewer players and isn’t quite as complicated. Basically, think of it as sort of like Team Fortress 2, but in 2D and with airplanes.
It’s also 50% off, making it $5. If that doesn’t make you want to at least try it, I don’t know what will.