Pyzor is a completely free database and software HashSharingSystem, written by Frank Tobin. http://pyzor.sourceforge.net/ .
*be sure to run "pyzor discover" after you install Pyzor, so that you can download the server to contact.
You can run SpamAssassin from the command line to figure out if it is using Pyzor. The process is described at RazorHowToTell, but the summery is to run the following from the command line:
spamassassin -D < /usr/share/doc/spamc/sample-spam.txt |
...where /usr/share/doc/spamc/sample-spam.txt is the path to a sample email that you want to test. Read the resulting debug text and you will know what tests get called.
*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/