As a service provider, I have tried just about every commercial and open source mail server out there. Surgemail is by far the best product I have used thus far. The feature set can't be beat for the price point. You get 95% of the features of CGP and Kerio for a tiny fraction of the cost. Every user has complete control over their accounts in the web interface. They can configure their own server side filters, spam filters, etc. The amount of control you have over mail services is amazing. It also includes a nice webmail client and a blog/file sharing service called SurgePlus. The only negatives are that the web interface is a bit clunky and it seems to crash once in a while. The developers and support team are responsive and helpful. Overall, you can't go wrong with Surgemail.

All the NeXTies out there will love this little utility, which restores the shelf from NeXTSTEP to Mac OS X.