Akismet : We can’t stand spam (in Blogs)

Who can? You have better things to do with your life than deal with
the underbelly of the internet. Automattic Kismet (Akismet for short)
is a collaborative effort to make comment and trackback spam a non-
issue and restore innocence to blogging, so you never have to worry
about spam again.
Akismet is free for personal use.

In the spirit of helping the blogosphere as much as possible, we've
decided to make Akismet free for as many people as possible. All you
need to start using it today is a WordPress.com API key. Sign up on
WordPress.com to get an API key.

http://akismet.com/