[watmough]

Voxatron.

  Frantic-ish action.
  Plays like Robotron. (hold down fire key to strafe)
  Voxel effects are awesome.
  Great sound, much less harsh than Robotron.
  Lots of power-ups.
  Very fun game.

I happily paid $10, as it's supporting both charities ($1.5 each) and the developer ($5.50).