Comment Referrers is a really nifty WordPress plugin developed by Donncha O Caoimh that does a small but useful task. It checks where the people who comment on your blog come from. It then adds a line to the end of your comment notification or moderation emails with that information.
Simple, yet useful. :) I wish it was possible to check if a comment has been left by a spammer or a real reader. [addition: Never mind! The spam posts are quite clear when you read ’em so there is no need to add any special function in the plugin to report spammers. ]