You can get LongURL as a standard Firefox add-on or as a Greasemonkey script.
If you don't use Firefox, you can go to LongURL.org and paste any shortened URL to see the expanded URL.
UPDATE, 6 May 2013: The LongURL Firefox add-on is no longer being developed and will not work with current versions of Firefox. The best alternative I've been able to find is unshorten.it for Firefox and Chrome. See also my How to Preview Shortened URLs (TinyURL, bit.ly, is.gd, and more) article which is updated much more frequently than this one.
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