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.