Simple Trackback Validation provides a simple check which will get rid of a very, very high % of trackback spam.

The Simple Spam Filter allows you to put a straightforward limit on the number of links allowed in comments – a great way to swiftly eliminate the vast majority of comment spam. AVH First Defense Against Spam blocks spammers before any content is served by identifying them at the Project Honey Pot, a local blacklist or the local IP cache. It also checks, at stopforumspam.com, visitors trying to post a comment.

And last, but certainly not least, Peter’s Custom Anti Spam Image Plugin provides a CAPTCHA solution that really works! (and all you have to do is edit the file to replace it with your own fun words)