SpamSieve is a powerful, popular spam-filtering app that can help you greatly reduce unwanted e-mails in Mail, Entourage, MailForge, Outlook 2011, and other e-mail clients. Because SpamSieve uses Bayesian filtering (a process that looks at the statistical probability of every word in an e-mail) and you train it using your own e-mail, the app actually becomes more effective over time. It learns not just how to better identify ever-changing spam but also how to zero in on the peculiarities of your own legitimate e-mail (whether that's using the names of your family members or your friends' frequent and obscure references to the "Halo" backstory).
SpamSieve integrates directly with your e-mail client, which is especially helpful if you use multiple clients or need to move from one client to another. You train SpamSieve using easy "Train as Good" and "Train as Spam" commands (from the menu bar, or with a keyboard shortcut, much like Mail's "Mark as Junk"), although when you initially set up the app, you'll need to make several changes manually in your client's preferences. Fortunately, SpamSieve has a clear and extensive manual, which even includes instruction on how you can use SpamSieve to help filter spam on your iPhone, iPad, or iPod Touch. For SpamSieve to be effective at first, you also have to spend a few minutes training it by hand with 100 or more messages. Combined with SpamSieve's price tag, this might be too much involvement for casual users, but for anyone who deals with a lot of spam despite their client and ISP's built-in filtering, it can be time and money well-spent.