View Thru is a neat extension for Google Chrome that lets you instantly see what's hiding behind a shortened link. You simply hover over the link (say, http://is.gd/bJoc7) and get an instant tooltip showing you where it's leading. When I tested it, the tooltip really was instant -- I did not have to wait at all. I tested it both with TinyURL and is.gd, and it worked equally well on both.

The shortened URL must be an active link, though -- it doesn't work when it's just text on the page. View Thru has earned a five-star rating in the Chrome extension gallery, and I can see why.