Junkeater – We fight guestbook spam: Wonder if this would work for blog comments? I mean, who has a guestbook anymore?
“If you are a webmaster and use a guestbook or automatic link page on your website you are most likely very familiar with guestbook spam: People who ‘sign’ your guestbook without ever looking at your page, just to leave behind a link to their own commercial page. We got tired of these entries and started looking for a solution against it — but we couldn’t find one. So we developed our own. And we decided to share it with other webmasters: This is how junkeater was born. “
I read somewhere once that Google filters guestbook spam links out of its pagerank algorithms anyway.