Gmail just rolled out a ton of new experimental "Labs" features. To see what they've added, go to "Settings" and click on the "Labs" tab. The addition of the classic snake game is very cool. The other two features I find especially useful are the Signature Tweaks (floats your sig above the quoted text) and Email Addict, which encourages you to take a quick break every 15 minutes.
May of similar features could be attained by using Greasemonkey scripts or the Firefox add-on Better Gmail. It's always nice when Google integrates user-created features so you can use them anywhere.