Here, from Wikipedia you can read about game which were commercial, and now they are freeware. Some of them are redistributable yet, but little number of them aren't. Link: http://en.wikipedia.org/wiki/List_of_commercial_video_games_released_as_freeware
Incomplete :)
Albion: Do you know Avatar? Surely. Do you now where its idea seems to came from? Try Albion ...
TES Arena: Now freeware but but still one of the best RPGs.
Both games run like hell in DOS session.
yap :) Wikipedia doesn't know everything ;)
More is Psion Chess old,but good 2D and 3D chess