Ever heard of Eve online space sim MMORPG? If you happen to run Linux or Mac, you're out of luck. The thing is Win-only and even the servers run on Windows. So the Linux client is highly unlikely. The same applies to some other popular games like Entropia Universe or Star Wars Galaxies online.
So - what's left for a Linux/Mac geek to have some fun? Well, you can try Vendetta online - http://www.vendetta-online.com (BTW - Windows version's available, too). The game has been in development for some 4 years, the "pay stage" has been a year or so. The community is rather small but responsive. Don't expect thousands of players online - you should expect rather tens - the total number of active players is somewhere around 300-400.
How does it play? Pretty well - you select a nation (Itani/Serco/UIT) and you are ready to go.
The skill system is simple but efficient - you have five different areas you can improve: Combat/Light weapons/Heavy weapons/Trading/Mining. You improve by using these skills and by completing special missions. So there is no "automatic learning mode" like in Eve, everything is hard-earned.
You can choose your fate - improve your stats in one particular area (though the first three are somewhat dependent). And as you progress you can buy better ships and equip better weapons - you know the line...
There are updates to the game engine (almost) every saturday. The game interface and functions have changed quite a lot since I subscribed. We even were blessed by robotic pirate ships in the "grey space"!
And more is to come - the devs are planning a real economy, creating introductory missions for newbies etc. The game is 10 USD/month with some discount if you subscribe for longer periods (6 months is 49 USD). You get free 8 hours online to try the game and I can guarantee you won't want your hard-trained character to disappear.
And finally some screenshots.