StumbleUpon's Su.pr URL shortener now open to the public
But there are few things that set Su.pr apart. First, like Bit.ly, Su.pr offers analytics that let you know how many times users have clicked on your links. And second, Su.pr is run by web site discovery service StumbleUpon, and every time you shorten a link, you also add a web-based StumbleUpon toolbar to the link that lets users vote on the story. In other words, when you shorten a link through Su.pr, you're not only able to share it with the few dozen friends who read your Twitter updates, but if they like it enough they can vote it up and brings thousands of StumbleUpon visitors.
Up until recently, the service was only available to a small group of beta testers. But today Su.pr dropped the beta label, and with it the need for an invitation to use the service. Anyone can use Su.pr to shorten a link. But you can also login with a StumbleUpon account to keep track of your data and if you want to post updates directly to Twitter using Su.pr, you'll need to enter your Twitter login info.