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Just wanted to bring up a frseh new thread.
Unity
http://www.unity3d.com/
Mac only but publishes to all platforms
Cost = $1000's
Torque
http://www.garagegames.com/
Mac PC Linux development for and on all 3 platforms
Cost = $100's - $1000's
Croquet
http://www.opencroquet.org/
Truespace? (colaborative mode)
http://www.caligari.com/products/trueSpace/ts7/Brochure/collaboration.asp?Cate=BCollaboration
http://metaverse.sourceforge.net/
http://www.imvu.com/
http://sociolotron.amerabyte.com/website2/index.htm
http://www.there.com/
http://www.virtualworldsreview.com/
http://www.activeworlds.com/
The last one being one that got picked over SL for Stage Coach Island (note for final use, could they really have done it without our community to help them find the flaws?

any more for the list? Discuss!