- SuperTuxKart is a fork of TuxKart - in terms of quantity of content and it's looks, the fork is for the better. I can't say anything about the other aspects (controls etc), as I never played TuxKart. I couldn't compile it.
- Speed Dreams (formerly known as TORCS-ng) is a fork of TORCS - so far I can only say that Speed Dreams is better in terms of technical project management (it uses Trac) and game menu visuals (compare these videos: SD - TORCS). I didn't look closely at content and controls.
Do you know of any more free software games that are forks of others?
PS: Quake engine X is a fork of Quake engine Y etc. (see quake engine family tree) let us.. just ignore them