The "main" site for rules is generally http://www.rulesemporium.com/, and spefically the http://www.rulesemporium.com/rules.htm page. They have descriptions for what they do. You'll also find more on http://www.exit0.us/, including the RulesDuJour script at http://www.exit0.us/index.php?pagename=RulesDuJour that will automatically check for new rules for you.
Don't forget the list of rulesets in the official Wiki:
http://wiki.apache.org/spamassassin/CustomRulesets
Really, I would regard the wiki page as the best starting page, as it references all the others, including exit0 and rulesemporium.
Rulesemporium is the outlet for all the rules built up by SARE, who do by far make the most and best add-on rules, but they aren't the only ones..
