A plugin which identifies and blocks comment and trackback spam on blogs with integration to various blogging systems... 

http://akismet.com/

An open source a PHP-based solution for blocking link spam and the robots which deliver it by analyzing the delivery method as well as the software the spammer is using instead of looking at the content of potential spam... 

http://www.bad-behavior.ioerror.us/

Provides an open source antispam plug-in for Movable Type and WordPress... 

http://antispam.typepad.com/