While I find Gmail's spam filter pretty adequate, for people with POP email accounts, or even worse, POP accounts that have to be visible on the web (like my techspot.com address), fighting spam can become quite the nightmare. Even with some server-side software setup, spammers can learn the software's filtering behavior and bypass it easily. Just the same thing happens with blacklists.
Say hello to POPFile, an open-source automatic mail classification tool that just works (after some training).
Read the complete post on the TechSpot Blog.