One of the best games ever created: Escape Velocity. It's made by Ambrosia Software, and it's pretty cheap (20-30 bucks).
http://www.ambrosiasw.com/games/strategy.html
Basically, what you do is start off in a galaxy in a small cargo ship and do whatever the hell you want. The controls and game looks something like Asteriods (think Metro's Armada) but better graphics, of course and hundreds more things to do. You start off doing oddball jobs like shipping cargo or carrying passengers from solar system to solar system, but as you gain more money you can upgrade your ship or buy new ones. Eventually you get to a storymode and play from there or you can still do whatever you want become a pirate, bounty hunter, or taking planets over... or just a really successful delivery service.
There are a whole mess of mods for this game, so you can play in a Star Wars or Babylon 5 universe. You can buy just about every ship you want (to get Imperial or Rebel ships you need to work for either side) from the movies or books. My personal favorite was a Star Destroyer, complete with fighter bays. Coming out of "hyperspace" and unleashing 50-80 small Tie Fighters (or many other small fighters) from your bay to attack the enemy (who do likewise with their own ships) and pounding the hell out of them. It gets very intense.
It's a dynamic world, so you can be minding your own business and jump into a solar system in the middle of a huge battle that isn't scripted. Alliances change as solar systems are taken over, and if you're from an enemy faction (or a pirate/smuggler) they will send out ships to attack you. You can buy fake IDs to get passed them, or if you're powerful enough (you have a ship with enough armor/shields and weapons and/or fighters) you can wipe out their local navy and take the planet (solar system) over or fight/run until you have enough energy or out of range from the planets gravity field to jump to the next system. Economies also change so you can invest in materials to sell/buy.
Depending on the mod (or the main game) that you're playing, there can be hundreds of systems to visit, each with their own planets, outpost asteroids, satelites, etc etc.
I've spent hundreds of hours on just Escape Velocity: Override. There is a newer version called EV:Nova, but I don't know if the Star Wars mod for it is done yet. It's an extremely satisfying game (unlike MMORPGs). Play the trial version (30 days free, i believe) to see if it's something you'd like, you can even use the mods with it.
