Steel Storm - A TPS Game For Windows, Mac and LInux

Steel Storm is an old school TPS game with score oriented competitive gameplay, and is designed for people who like fast paced action and have limited amounts of time to play computer games.

The events take place in an alternative universe where you control an advanced hovertank, packed with the most advanced and sophisticated weapons. Your task is simple, but, nevertheless, is not trivial. In the fight with presumed extraterrestrial invaders, you must prevail!  (via game's page

The beautiful graphics, the background music is also good, and it's simple to get start, just use WASD keys and mouse to complete control. In a word, it is a nice game for killing time.
At the moment the developers have released episode I as a free title and the episode II as a commercial product.

In addition, I found a Steel Storm in FPS mode video on YouTube, maybe the gameplay could be change into FPS in the future;)

Downloads |  (About 300MB)  Linux users just extract the downloaded package and click on either the 32-bit or 64-bit executable depending on platform and away you go.