Here are a couple of interesting spamassasin rule sets:

• Jennifer's Sets
• Rules Du Jour Rule Sets
• Rules Emporium

Here is some related help...

• "Installing and configuring Exim 4 on Debian" - Nice step by step how-to.

Here is something that made a huge difference on my spam set:

• SURBL - Spam URI Realtime Blocklists

  This is similar to an RBL, but instead of looking at an email source it looks at URIs found in the email body.