After a small amount of time some games go from a commercial license to another license - usually an open source one. Once games fall into the open source type of license then they usually get modified by the gaming community and made into better versions. These are then usually released on developers websites.
One game that has really been modified a lot and released - which I remember playing this game years ago and loved it - was Warzone 2100. I used to play it on PSX ("Playstation") and loved the game, then when I found out - via WikiPedia - that the game was made Open Source and a community decided to make it better, I knew I had to get it and play it. It works on XP, Vista (and [I think] W7).
Warzone 2100 isn't the only game that is now given out for free. You can also get hold of Command and Conquer, that was released 31st August 1995. The game comes complete with Disc One and Disc Two in the form of ISO files (so they can either be burnt to a disc and ran or mounted using something like PowerISO or MagicISO).
Also another few old games are also available for free download. Rockstar (yes, the GTA guys) have also made Grand Theft Auto, Grand Theft Auto 2 and some other game called Wild Metal free to download and install.
Who knows, give it another few years and other big titled games might be available.