Searchengineland has a useful post on URL shorenting services, Analysis: Which URL Shortening Service Should You Use?.
“URL shortening services are experiencing a renaissance in the age of Twitter. When every character counts, these services reduce long URLs to tiny forms. But which is the best to use, when so many are offered and new ones seem to appear each day? Below, issues to consider and a breakdown of popular services, including recommendations and services to avoid (the new DiggBar being one of these).”
They review 15 services and discuss some of the underlying issues (e.g., 301 vs. 302 redirects).