TF2 (Team Fortress 2) is a great, free to play, multiplayer FPS (First Person Shooter) with a long history. My real interest in it is the art direction; which apart from a gorgeous, highly stylised design aesthetic carries over wonderfully into the various 'promo' videos which introduce each character. They are cartoonly violent, dark humoured and a stand out example of how to holistically create a video game universe and experience... and so enjoyable to watch.
The Pyro (above) has just been released but check out the other videos.
Also, have a look at this interesting and powerful open source movie maker used by Valve to create these movies!