As everyone above suggested, bit.ly is probably the best one available in the market. But I will suggest you to take a look at a new link shortener service called Snip.ly. I started using it very recently and really loving it.
The uniqueness of this link shortener is that you can add 'custom call to action' to each shortened link. Then you can track analytics for both the link and also the 'call to action' engagement. You can give it a try and see whether it serves your purpose.