It’s hard to have a market with only one player. Though web spam is a huge problem touching countless millions of people, there are still only a few ways to stem the tide. Outside of Akismet, almost all solutions thus far have been user-hostile, like CAPTCHA, not adaptive, like all the one-off solutions people do, or falsely assume that web spam is anything like email spam. (There was briefly an Akismet clone called Linksieve, but I can’t find it on the web anymore.)
Today we would like to welcome a new entrant to our nascent market, Defensio. They have a similar approach to us, with a centralized API that gives a thumbs up or down for a comment, and some added information indicating “spamminess.” Best of luck, guys!
(And for those wondering, Akismet only returns “yes” or “no” to avoid giving spammers any additional information. However the interface for managing spam could definitely be better.)