Multi User Virtual Environments (MUVE's)

Active Worlds EDUcational Universe (AWEDU). A showcase world of Virtually Learning at http://www.virtuallylearning.com/AWEDU.htm

Open Croquet http://www.edutechie.com/2007/07/7-ways-croquet-is-better-than-second-life/ at http://www.opencroquet.org/index.php/Main_Page

iVersity at http://www.iversity.net/ at http://www.iversity.net/index.php?option=content&task=view&id=20

Multiverse at http://www.multiverse.net/

Protosphere http://protonmedia.com/