Thousand Parsec is a 4 X's game (eXplore, eXpand, eXterminate, eXploit) but it is different in the sense that it is designed for long games supporting universes as large as your computer can handle. (Have a quad Xeon with 4 gig of ram? Well have a couple of million systems in your universe! :). It also supports long term play with players joining and leaving throughout the game. As well the tech tree is easily expandable and can be extended during the game without effecting anybody already playing (ie introduction of new technology as the game progresses).

License: free