Second life is tangentially an example. they open sourced (or similar) the official viewer and some enhanced community and custom clients are out there like fir

Second life is tangentially an example. they open sourced (or similar) the official viewer and some enhanced community and custom clients are out there like firestorm oh, and quake 1's quakeworld client. many excellent quakeworld clients like darkplaces and ezquake. lots of one offs showing off tech, etc