SquidGuard is a URL redirector used to use blacklists with the proxysoftware Squid. There are two big advantages to squidguard: it is fast and it is free.
DSPAM
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).