Planets is a fun, interactive program for many-body gravity simulations. The emphasis is on play: it's easy to add in planets, zoom in and out, change the physical constants, save and load configurations, etc. It's designed to be easy enough for a kid to enjoy, but that hardly precludes adults.
Free Tennis is a tennis simulation developed by a former tennis player. Its main feature is realism. For gameplay, "realism" means you have total control over the shot parabola. For graphics, it means players have realistic gestures. For AI, it means real tactics. A two-player mode via LAN is available.
Goba (Game Of Bases) is a remake of the classic Flash Attack game featuring pre-rendered graphics, sound, and networking (no computer opponents) with meta-server support. You control four tanks and try to destroy enemy bases while defending your own. The tanks have weapons, a set of mines, and the capability to build walls and fake bases, all of which may be recharged back at the base. The bases have a limited number of bomb attacks in their disposal, plus a laser beam that can cut through the whole game field (2000 x 2000 pixels).