Using Pyzor
Pyzor is a completely free database and software HashSharingSystem, written by Frank Tobin. http://pyzor.sourceforge.net/ .
A documentation how to integrate Pyzor into SpamAssassin can be found on http://www.konabi.de .
Pyzor on Debian
If you are using Debian, there are (currently) packages for testing and unstable available from the main tree: http://packages.debian.org/cgi-bin/search_packages.pl?keywords=pyzor&searchon=names&subword=1&version=all&release=all
There are also backports to woody available from various sources. One (that works for me) is: http://www.gothgoose.net/debian/