Gmail Labs is packed with goodies, including some that look pretty useful and others that are really whimsical. I'll just mention two, but do check out some of the others.

Mail Goggles (no, that's not a typo) makes you solve a few simple math problems before you can send an email. The premise is familiar to anyone who has ever sent -- and later regretted - an email when they were really tired or drunk. If you're too groggy to pass the math test, you should go to bed instead of embarrassing yourself. You can set Goggles to pop up the safeguard at whatever hours you choose.

Another applet links Gmail to Google Translate, while others do all sorts of stuff from inserting images or maps into an email to playing goofy little games.