I like URL X because the shorten URL's it makes are still human readable and there's a very simple API. However its been shutdown again.
Bad news: Dreamhost has shut down url(x)! (Again.) They don't think the service should exist in its current anonymous incarnation. I disagree with their assessment and will be moving the site to a new server. I will be tightening some things up, but the service will return in its current form. Sorry for the inconvenience — we will be back shortly!