I just enabled the "nofollow" tag on the website URLs commenters leave. Now, when Google announced "nofollow" as a way to deter spam, I knew it wasn't going to be effective at getting rid of the stuff. And I was right. Matt as WordPress, which jumped on the "nofollow" bandwagon with a lot of flag-waving and ticker-tape parades, had to create Akismit to help with comment spam.
If "nofollow" was effective, we wouldn't need all these alternatives. But we do.