Web site (and Firefox extension) LongURL expands URLs that have been shortened by services like TinyURL, Ping.fm, is.gd, and tons more, so you know where the link is pointing before you follow it. The site itself is a decent tool to begin with, but the Firefox extension is what makes LongURL really useful. Once installed, the LongURL extension will automatically expand a shortened URL in the tooltip when you hover over the link with your mouse (it doesn't waste bandwidth until you hover over a compacted link). Since it uses a web service, the supported services are automatically updated when new services come and go. LongURL also comes in Greasemonkey script and Ubiquity form, so be sure to check the tools page for the add-on you prefer. While you're at it, check out previously mentioned Embiggen and Tin Foil Hat.