I don't know that I would consider them compitetors... but if you will,

Deklarit
Enterprise Library
CSLA.NET
http://www.gavinjoyce.com/
http://www.devigus.com/Downloads/VDX/VDX.aspx
http://www.westfaro.com/

Just to name a few - none with the functionality that MM.NET offers. Most are just business object frameworks or ORM Frameworks but no rich UI feature that interact seamlessly with the B.O. layer.