I guard my personal email addresses like they are my children, but every once and a while a little spam still sneaks thru. Mail.app does have some junk mail software, but it never hurts to have more. Especially one that is well built. JunkMatcher is a very good solution that integrates tightly with Mail.app.
From the Developer's site:
It filters spam by conducting a wealth of tests over emails. Two types of tests are available: property tests can detect general characteristics of spam using techniques such as naive Bayesian filtering (thanks to SpamBayes) and blacklist lookup (checking if a message was sent from a spamming IP address), and pattern tests can spot keywords such as ?v1ägra? or ?/Iagr á? by using patterns written in regular expressions, a very powerful language designed just for that - to describe patterns.