What would prompt you to use Su.pr over the slew of URL shorteners already floating around? In addition to basic URL shortening, Su.pr offers analytics to provide traffic data about your submitted links and every link you submit to Su.pr is added to the to the StumbleUpon database.
If you use the "post to Twitter" function you're simultaneously shortening the URL and sharing it with the Twitter audience and the StumbleUpon audience. Whether or not you're trying to drive traffic or just share something you think is awesome, it's a great way to get more eyes on the site you're sharing. Simply shortening a URL is available without an account, to take advantage of the more advanced features you'll need to sign up for a free account.
Su.pr [via DownloadSquad]