I just read a review of five URL shorteners and added a comment recommending the one I'm building my systems on, Adjix.
Adjix is imho, the best because it automatically stores each link it shortens in an Amazon S3 bucket that I own. If they ever go away, or if for some reason I decide to move on, my links won't break (at least not because of them). I also use a custom domain for my URLs. So I get complete data portability at no cost. Absolutely no lockin.
Side-note -- this raises another question. A lot of us use Libyan domains for our shorteners. Have you been reading the news from there? What if they figure out that they can cause a major disruption on the Internet by breaking the deal. Brrr.
I know a lot of people haven't heard of it, but it's really important. Someday a lot of these links are going to break. I'm doing everything I can to be sure that mine last. I know other people care about this. Adjix has made it work.