The newer version of .NET based RulesEngine has been launched by ObjectConnections. A comprehensive RulesEngine provides a seamless and efficiently architect product providing scalable solutions for the business. This one seems to be new and is downloadable from here.
The other good RulesEngine is from Yasu Technologies. I have been using this in my previous organization and is quite friendly. The tech support is also quite fast.
There are a few good open source rules engines too. A few of them are: