There are dozens of URL shortening services available, most of which are the same in terms of features. Some, like Bit.ly, differentiate themselves with stats and such, while some others do so with their size (no pun intended 
).
u.nu is one such URL shortener – it has the shortest number of characters in URL possible. There are other services with three letter domains, but u.nu also has a convenient API etc. for use.
Usually, API is used by services that interact with the site, but u.nu API is simple. To create a new short URL, all you have to do is, use the URL http://u.nu/unu-api-simple?url=<YOUR_URL> where YOUR URL is the original, longer URL. More details about the API are available here.
You can also drag the bookmarklet and place it on your bookmarks toolbar for easy access.