goo.glThere are a lot of URL shortener's available, but I think that goo.gl - Google's URL shortener, is the best and has the most features.
It is available in a variety of formats:
Standalone web page - enter the url and shorten it. If you are logged into your Google account, it will keep a record of the pages you have shortened, the original URL, and even how many times someone has clicked on that shortened URL. This is a great way to track URL's that you are sharing and seeing how many people click on them.
Chrome Browser Extension - adds a button on your Chrome extension toolbar that let's you have one click access to a shortened URL for the page you are on. It will also create a QR Code for that URL. It also adds the URL Shorten feature to your right click menu while in Chrome, allowing you to easily shorten and copy a URL just by right clicking on a link in a webpage. (It works great in Chrome OS also). You can also share the link with different services like Blogger, Evernote, Facebook, Twitter, and many more. It also captures data and tracks clicks.
Firefox Extensions - there are two Firefox Extensions that use goo.gl.
Copy Goo.gl URL and goo.gl Lite - both do the basic goo.gl functions.
Google Toolbar (for Internet Explorer and Firefox) - goo.gl is available on the Google Toolbar (in fact, it started out there before getting to Chrome) and has the same features as the Chrome version.
Google Toolbar is also an excellent resource for people who use Internet Explorer or Firefox, bringing many different Google tools and features to those browsers, including search, sharing, translate, and more.
I recommend goo.gl URL shortener because it is easy to use, rich in features, and works.