[welterde]

Warzone2100 would be a proper example of an commercial game having a open source afterlife (including being ported to linux/mac and continued development to this very day).

[ghostDancer]

Freespace 2 SCP/FSO, they have made mods, add better graphics and have a nice community going strong. http://scp.indiegames.us/ http://www.hard-light.net/