Drools.NET is a port of the Drools library to .NET. I have a bg, big architectural decision coming up for a client and I am debating about whether to tackle the issue with an inference engine or a scripting language. So I've been looking at Drools pretty closely. It's okay. I wouldn't put it in the same league as ILog JRules, but the price is right and it seems to have momentum.
In the .NET front, I've been told there's a Rete-based inference engine inside BizTalk (?), but I never followed up on that. Another tool is mTuitive's xPert Authoring Environment. More to check out....