Some games, yes. Epic foster an incredible community with their incredibly generous terms for licensing the UDK out to all. Skyrim will no doubt see a profusion

Some games, yes. Epic foster an incredible community with their incredibly generous terms for licensing the UDK out to all. Skyrim will no doubt see a profusion of mods; the Total War games all have a healthy modding community; all Valve games are as moddable as HL2 was, given they... uhh... are still on the same engine.