Many other rulesets exist and are published on the Internet. These are often themed, for example, detecting invalid HTML, common URLs used in spam, or catching sequences of characters often used in spam emails. Custom rulesets are often updated frequently in response to changes in spam being sent.
An Internet search for 'spamassassin rulesets' will return many pages linking to rulesets. The SpamAssassin wiki (a collaborative information website) includes links to many custom rulesets on http://wiki.apache.org/spamassassin/CustomRulesets.
Custom rulesets may have specific installation instructions that should be read and followed. In general, the installation involves copying a rule file and a score file into