Sarkscape, the creators of Ninja Ropes and Block2Lock have created a great, asteroid-like game called Mass Defense.  In the game your objective is to make sure no planets hit yours by shooting at the and redirecting their course.  The white meter on the top left indicates the amount of ammo you have and the red bar is health.

I really like this game and played about 5 rounds straight just now.  I'm finding the key is to let your ammo build up as much as possible and try to get your shots to bounce from planet to planet so you get more bang for your shot.  You'll definitely get hit at some point so make sure the small planets hit you and not the big ones.