CommentLuv is on sale today. This is one of my favorite plug ins.
There is a free version but the paid version has a lot more to it. I broke down and bought it a while back.
It has an anti spam section you can use if you want to as well as twitter links, the ability to allow people who comment to choice which of ten posts they want to link to, and the ability to make the links nofollow or dofollow.
It also has a new section that allows emails to be sent when a reply is made to a comment.
If you have ever thought about getting the paid version you should do it now…while the sale is still on.