Hi
I would recomend you to try the game EVE-Online wich you find on
www.eve-online.com where you can download the game and get a free trial (14 days) to check it out.
The game is for Windows but have heard people play it on Linux among others...
Some even experiment with it on Cellphones... well remote windows or something hehe.
I have played this game for like 3 years now and it is great, one server... from 25000 to 33 000 players online at the same time, you can be a smuggler, trader, producer, fighter, CEO for a small corp, big corp alliance with thousands of members... even if you like so you can be a Pirate :-)