Didn't see this on the page, but not sure if its truely freeware as its based off of and needs some files from a commercial game (although they are easily available online through links in the sites forums.)
But
OpenTTD has grabbed some of my time lately. Its an open source version of Transport Tycoon Deluxe. I could never really get into the game when it first came out but have really started to get addicted to it this time through. Theres also a link to TTDPatch which is another version/branch that has some pretty neat new features.