Michael Arrington blogged about Akisment filtering out 15,000 SPAM comments per day (!) on their site.

I'm wondering why CAPTCHA is not a better solution.

a) It doesn't require a server-to-server call to validate the comment.
b) Is more reliable as a "real-human" filter.

Can Akismet really stop someone determined to post "spam" on his site by hand, anyway? Spam is in the eye of the beholder. I don't know what Akismet's algorithm is, but it seems to be it's a huge scale issue to have all blogs use a centralized service to perform this function.

Akismet is tailored for Wordpress comment spam, but can also be used for other forms on online posting. There's a developer API here:

http://akismet.com/development/