SimCity 4Andy Koch sent me some information about an add-on he's developed for SimCity 4. He explains it better than I can, so here he is (slightly edited):
Maxis' SimCity Scape lets everyone registered play a region cooperatively. People register, and get a city for a term - like 5 days. Then you upload your work back to their server and in this way everyone contributes to the overall region.
Back in January myself and a couple of friends began a project named SimCity Online Region Exchange - or SCORE for short. It does what SimCity Scape does but it's open source - so anyone who can figure out how to setup apache and php can host their own coop region games.
If you'd like more information, go here: http://sc4ore.sourceforge.net/ .