LATEST UPDATE 2011 : bit.ly Use *this* tool for twitter marketing even without follower! bit.ly
While there are many many url shortening sites out there, there are also some sites that twitter puts up a warning on. So before you use it because of the tracking features make sure that twitter won't put up a warning when your followers click the links!
For further information check our main page at http://twitter-biz.com for some great twitter for business information.