It would be better if you created your own MMO
There are plenty of good engines out there, Mainly for 2D MMOs, But there are a few I know of that are for 3D.
Mirage Source (VB6) -
www.key2heaven.net/ms/forums
Most likely one of the best 2D ones out there, its only got basic features, But its practically bug free, and is open source. So you have alot of coding opportunities
vbGore (VB6) -
www.vbgore.com
Very powerful, bug free engine. Kind of complicated to use, and requires a MySQL database. I've only used it a few times but was impressed with what I used.
Elysium Source (VB6) -
www.elysiumsource.net/community
Many versions of this engine. It is basically Mirage Source but with many many more features. Though it is more buggy. If you were to use this I would reccomend using either Debugged or v3.0 - v2 is realllyyy buggy.
Crystal Space 3D (C++) -
http://www.crystalspace3d.org/main/Main_Page
Crystal Space is a pretty cool engine. Used to make the game Planescape.
Its 3D of course, and is fairly easy to use. There are also many tools developed to aid you in the design of the game.
Playerworlds (VBscript) -
www.playerworlds.com
Its another Mirage Source based engine, But this one is closed source. The person who is the lead programmer of it is doing a really good job of the new version, and when it comes out it will be great (I reccomend this one.)
It has a powerful scripting engine which is all you need to add features into your game.
They are all the engines I know for now