SearchEngineLand & NotLong have compared popular URL Shortener services.
Google offers its own URL Shortener service called goo.gl (gl is the country-code top-level domain designated for Gibraltar). It now also has an API.
Pingdom, the uptime & performance monitoring service, has found goo.gl to be one of the fastest and with 100% uptime.
For spam, security or legal reasons, the goo.gl service can disable some short URLs