Red Orchestra: Ostfront 41-45, a realistic WWII first-person multi-player combat, is available on Steam for Linux and it can be purchased with an 85% discount.
Red Orchestra: Ostfront 41-45 was initially developed by a team of modders, Tripwire Interactive, that won the Make Something Unreal contest.
It aims to be one of the most realistic WWII first-person experience and incorporates a lot of nifty features such as a ballistic system, realistic armor properties, and historically accurate units, such as tank, planes and infantry.
The hardware requirements aren't all that high. Here are the recommended Linux system requirements:
• OS: Ubuntu 12.04 LTS, fully updated;
• Processor: 2.4 GHZ;
• Graphics: Nvidia, ATI, or Intel GPU with hardware-accelerated drivers;
• Hard Drive: 2 GB free hard drive space.
If you are interested in this FPS, you should check the official Steam website.