here are some of the free or open source .NET Business Rule Engines I have come across

NxBRE (port of the Java JxBRE)
http://www.agilepartner.net/oss/nxbre/
http://sourceforge.net/project/showfiles.php?group_id=97759

Jaxlab Rules Engine
http://www.jaxlab.com/rulesengine/default.aspx
http://www.codeproject.com/useritems/Rules_In_Your_Apps.asp
http://www.codeproject.com/useritems/Using_a_Rules_Engine.asp

Simple Rule Engine - SDSRE
http://sourceforge.net/projects/sdsre/
http://thusithata.spaces.live.com/Blog/cns!6B1F9DE12D24534D!131.entry