Battle for Wesnoth (turn-based strategy)
People would be paying good money for this. It has multiple quite long campaigns. Decent artwork and dialogue as well. If you like Advance Wars and such, you'll like this.
Neverball (Super Monkeyball clone)
It's a 3D physics puzzle platformer and a sort-of racing/speedrunning game in one. Instead of controlling the ball directly, you tilt the level. Works amazingly well with a mouse, probably better than the Monkey Ball analog stick control scheme.
XMoto (2D motocross)
Similar to Neverball, it's a skill-based physics platformer, but in 2D, and also encourages replaying to improve your times.