I'm currently putting together a report for a client on virtual world technology. Searching through the noise I've come across a couple that look good (including the obvious Second Life):

http://www.smartfoxserver.com/ - Can create 3D worlds through Flash

http://www.multiverse.net/ - A good development platform but not quite ready yet, I don't think

Does anybody know of any others that might be of use? I've searched long and hard and can't find much that I like.

Also...while I'm at it, does anybody know how many players can realistically be supported in the Source Engine? I know the "maximum" is 32, but is that just a hardwired restriction or can it be revoked?