Spicing things up a little bit, ArmoredCore.org has added a great multiplayer, turn-based browser-navigated PHP game to the arsenal of good things here.  The main premise of the game is that the player is given command of a ship in the galaxy, and can earn money by trading between the different ports in the galaxy.  After making some initial money, the player can colonise planets, create an empire, form alliances with other players, and wage war with other players!  It's tons of fun, and you can check it out and get started on your empire here:   http://www.armoredcore.org/bnt55/

For an FAQ on the details of the game: http://www.armoredcore.org/bnt55/faq.html