This physics game uses both the keyboard and mouse, and it requires some getting used to. But once you play through the first few levels, you might be addicted. Levels 20-30 are definitely my favorites, they use more letter cannons, and they are more puzzle based. The levels get harder and harder to solve, but you can always skip levels after you complete a few in each series.
Update 2/16/11: Version 2 just released on Newgrounds with 30 more levels.