U3 is a consortium of hardware companies and software companies behind a standard which allows everyday USB memory keys to function as an individual’s mobile computing environment. If you’re on the road, and know there’s going to be a free computer where you’re going, you can just plug in your USB key and have access to many of your desktop applications already configured the way you like it.
It’s pretty cool stuff and they’ve already got a decent list of applications that are ready to operate on U3 based USB keys. I haven’t used a U3 key myself but I’ve played around with the portable firefox on a generic USB key and it worked nicely.
Right now I wouldn’t really say it’s a true substitute for my laptop but it’s a great backup just in case. In the future I’d love to have this type of functionality built into my phone. Rather than laptops or desktops we would all have universal docking stations where we can plug in our phones and get on with computing (and wirelessly using something like Bluetooth too).