Kerbal Space Program, a multi-genre game in which the players have to create their own space project, is the best-seller on Steam for Linux.
Kerbal Space Program is a game that's part of the “Early Access Game” initiative. Developers can sell their games, while they are still in development, and the users can participate by providing vital feedback.
The fact that the game is resting comfortably on the first position of the podium means that, despite being in direct competition with a lot of games that have been on sale until recently, quite a few Linux users have been very interested in it.
Kerbal Space Program features a physics-based Space Flight Simulation engine, the ability to fly to the Moon and other planets, the ability to build ships with multiple stages, and much more.
If you are interested in this game, you should check the official Steam website.