So, I just got a sneaky bit of comment spam -- so sneaky it's hard to tell if it is spam. The commenter is friendly, chatty, clearly read the post, but then leaves gratuitous links in the middle of it -- the links are relevant, but one in particular gets my attention -- it's a brand new commercial ::cough:: pseudo food blog -- I say pseudo because it's clearly set up to generate ad-click income more than anything else. Anyway... I googled the blog's address, found no legit links and instead, only links on other people's comment pages. Ahah!
Also -- another ahah! I discovered the html attribute (rel="nofollow"), googled that, and found this nifty entry in the google blog. Inserting rel="nofollow" into comment links causes those links to not get any credit in google rankings, therefore defeating the purpose of the spam.
Of course, the easiest thing to do is just delete the spam completely, but sometimes -- as with my most recent case -- the comment may be legit, and it's always nice to have an additional spammer-thwarting tool handy.
This trick has been around for a while, but I've not seen it before, and in light of other recent discussions on sneaky spammers, I thought I'd toss it out here. The google blog entry also contains a list of blog software makers that use rel="nofollow" in their comment coding. I am (for the moment) on an older version of MT (which was doing just fine till the new, paid version came out and the developer of MT-Blacklist stopped updating), but I'll be switching to Wordpress soon and embracing their built-in rel="nofollow" code, not to mention that nifty setting where you can refuse comments that contain more than two links. (Meanwhile, almost every one of my old comment threads are turned off, as the online poker fiends are out to get me in force!)