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.