Download here: getsubspace.com/ for free. Been around for over a decade and is still awesome.
I have to give you a fair warning, in the beginning it might feel a bit difficult to become good at this game and the people you'll meet as first won't always be as helpful to newbies. However if you continue to play for a few weeks, you'll discover this game has a lot to offer, different events within the game, leagues, tournaments etc, more than you can imagine when you first enter.
What has kept me going for so many years are the leagues and a community full with interesting personalities.
Also this game is the only game out of all the games I've ever played where it doesn't get extremely redundant! You know which games I'm talking about, WoW, Everquest, etc. Those games with millions of people are all about how much time and money you can waste! Not this game, the beauty of this game lies within two things: It's community and the ease of play.
An important tip: When you first load up the your resolution will probably be on a low setting, so click the resolution tab on the load up screen and select the highest resolution your computer will go to.
Note: The game was rewritten to eliminate cheat programs and is now known as Continuum as a result