Orbit-
http://digg.com/newsbar/topnews/Kill_some_hours_with_this_addictive_gravity_based_game
Planets are blue balls (heh...) and their size determines their gravitational pull.
The comet is just a small orange ball with a trail whose size is dependent on the comet's speed.

Galactic Gravity Golf-
http://www.gameshot.org/?id=83
Space golf game. The movement feels a little too fast. Nice replayability, though.
More complex art. 3D look to the planets with shading and textures.