While plenty of video games have taken place in outer space, few games really nail space combat on the head. A few older games like Tie Fighter and Wing Commander did a good job, and now there's a free game from Bigpoint that ups the ante.
Battlestar Galactica Online is an in-browser MMO game loosely based on the sci-fi television show 'Battlestar Galactica.' Don't let it's affiliation with this show throw you, because the back-story has just enough references to keep fans happy while not overwhelming newcomers. When you join, the first thing you do is choose between the two sides (Human or Cylon) and then create a character. Now you're in the game and ready to take on the enemy fleet.