In this game, which is something between Asteroids and Spacewar, I've implemented a bunch of space-themed sprites. There are dangerous asteroids that'll break apart when you shoot at them, stars that inexorably pull everything towards them, and, of course, your trusty spaceship that you'll use to navigate the heavens.
Gravity is pretty realistic between stars and other objects, but I plan on adding mass to asteroids and spaceships, so that everything would be affected by gravity realistically. Implementing mass for the spaceship will also open up the ability to use Newton's laws for propulsion, which would give another layer of strategy to the game: You need to conserve your fuel, and carefully mine more from asteroids to keep playing.
Though it does seem at this point that the game is leaning more towards Asteroids than Spacewar, I do plan on adding multi-player support, and, if it works logistically, online multiplayer through WebRTC or WebSockets.