If bit.ly wasn’t short enough for you, try j.mp. It’s a new domain owned by bit.ly. It’s just 3 characters including the top level domain. That’s pretty short. It runs the same URL shortening service currently being offered by bit.ly.
I have recently started my own personal URL shortener: uly.me. It works great, too. It uses base36 numerical system and generates sequential characters instead of random. It’s simpler. If you ever tweet, URL shorteners are a godsend.
By the way, I’m running a Twitter-based website on uly.me. It uses a PHP class which pulls Twitter information via APIs.