Scoregasm is a fun top down shooter by Charlie's Games. It's available for Mac OS X, Windows XP/Vista/7, and Linux. The game is set in a little galaxy made up of a network of different levels, which you progress through in a non-linear fashion, opening up new routes and levels depending on how well you play. There's multiple endings too!
The basic gameplay, if you hadn't guessed already, is shooting stuff! In addition to shooting you can use your Close Range Attack, a powerful energy blast that destroys things close to your ship.
Of course, you can power stuff up too! As you kill enemies you build up a combo. Get your combo high enough and your weapons start to power up by themselves, adding extra bullets and missiles to your arsenal.
In addition to building up your weapons your combo also lets you choose new pathways through the game. If you end the level with your combo above one of the exit targets the stage will extend into a Scoregasm Frenzy which, if survived, will allow you to choose up to 3 different exits, each taking you to a different level!