Chris Santerre has a collection of custom rules available at the SpamAssassin Custom Rule Emporium - http://www.merchantsoverseas.com/wwwroot/gorilla/sa_rules.htm
There's an additional SpamAssassin wiki at http://www.exit0.us . This seems more focused on rules. That wiki includes a German Ruleset.
Bill Stearns has a collection of spammer domain rules at http://www.stearns.org/sa-blacklist/ .
You might want to modify scores on existing rules, or write your own rules – see WritingRules.
See http://www.exit0.us/index.php/RulesBasics.
Where do I put rules ?
(copied from http://mywebpages.comcast.net/mkettler/sa/SA-rules-howto.txt )
~/.spamassassin/user_prefs
is best if you want to have a rule only run when a particular user runs SA. Bear in mind that this is the user that executes SA, not the user who the mail is addressed To:. If you run SA from a sendmail milter, the only user_prefs that will be used is the one for the user sendmail runs as (usually root). Even if you do have a site-wide configuration, user_prefs may be useful to you by allowing you to add rules to another user account's user_prefs and test them using the command line prior to adding them to your local.cf.
Note: if you use spamd, rules placed in user_prefs will be IGNORED by default.
Do NOT add your rules to the .cf files in
/usr/share/spamassassin
- When you upgrade
SA (and you should do so somewhat regularly), all the existing rules in
/usr/share/spamassassin will be deleted and replaced by the new default ruleset.
/etc/mail/spamassassin/local.cf
is the place of choice for site-wide application of a rule. Rules placed here get applied no matter what user invokes SpamAssassin.