AstroFire

Astro Fire is basically a run-of-the-mill Asteroids clone, but with a few twists: instead of asteroids you've got monsters of sorts to shoot at, there are power ups (two kinds of weapons) and shields (which means a single blow doesn't kill you), and you can also set whether or not you want momentum enabled, which gives an interesting edge to the game.

The full version of Astro Fire includes three different episodes with too many levels to mention.