I'm in a rush today - back to proper Free Gamer business as usual next week - so just a quick one.
The old school GTA games may be available as freeware, but purists will be happy to see Passenger, an open source game which shares many gameplay elements with the newer GTA games. Based on the Spectrum classic Turbo Esprit, the idea is to hunt down and kill terrorists rather than indulge mindless gangster oriented mayhem. The fact that you may cause carnage en-route is moot. :-)
Another Windows-only freeware game called Chaos, has a demo available (gameplay video). A game based on molecular simulation - if that's not original then I don't know what is! I am in direct contact with the author so will try to talk him into making it available under a Free license (think Linux port) so we'll see where that dicussion goes.
(You can always tell I'm rushing when I mention lots of freeware as I don't have time to research Free games. Since I'm rushing, no music tip today either! What a slacker!)