Tired of checking your gmail several times a day, especially if you have to go clear out the google cookies so they don’t tie your searching behavior to your mail? The Gmail Notifier will make your life a little easier, then.
So far, it’s Windows (2k/XP) only, but they say Mac and Linux versions are on the way.
Unfortunately, it uses the system "New Mail" sound, so there’s no way to have it unambiguously audibly inform me of my gmail.