Great stuff: http://expandurl.appspot.com/
Especially when a shortened URL breaks, and you want to find out if the underlying URL got moved to a different place.
--jeroen
via: http://stackoverflow.com/questions/6500721/find-where-a-t-co-link-goes-to
Edit 20240829
The above URL stopped working after a few years, so I dug up the history of it:
The Wiert Corner - irregular stream of stuff 