I’ve implemented Donncha O Caoimh‘s steps to fight comment spam here. Two of the plugins, Comment Referrers and Delink Comment Author add links to the comment moderation email I receive that show the referrer, which helps identify spammers, and remove links from the comment.
Lucia’s Linky Love makes this blog less attractive to spammers by adding ‘nofollow’ to their links, while rewarding regular commenters by making their comments ‘dofollow’.