DSpam - Content based Spam Filter

DSPAM is a scalable and open-source content-based spam filter designed for multi-user enterprise systems. DSPAM is an adaptive filter which means it is capable of learning and adapting to each user's email. Instead of working off of a list of rules to identify spam. DSPAM's probabilistic engine examines the content of each message and learns what type of content the user deems as spam or nonspam.

Source forge: http://sourceforge.net/projects/dspam/