If you are a twitter user, or regularly receive tinyurl or other short url you never know where these links will take you. LongURL wants to help you to expand these links and see where they are leading you before opening the link.
LongURL has simple interface, just input the short url and click expand to see where the link is leading. Yes it may not be feasible all the time to go to the site and input the links, LongURL has tools for that. LongURL Firefox Extension and Greasemonkey Userscript do the same as web interface does. After installing Firefox Addon just hover any short url and it will display the link as a tooltip.
LongURL supports tinyurl.com, is.gd, ping.fm, ur1.ca, bit.ly, snipurl.com, tweetburner.com, metamark.net, url.ie, x.se, 6url.com, yep.it, piurl.com and more.
This is a very useful service.
Inspired by LongURL, http://PrevURL.com gives a screen shot of the link that the shortened URL points to as well.