I have recently been bombarded with trackback spam here at my blog. After searching around the web, I decided to try out Spam Karma 2.0, a neat little plugin for WordPress. I have only used it for a very short time, but it's already filtered out the spam I was seeing! If you are having problems with spam on your WordPress blog, I highly recommend this plugin.
As a bonus, all first-time comments will be viewable immediately (they no longer go to moderation).