The U3 website is one of those hardware industry initiative sites so overdesigned and inscrutable that I can't tell if it's promoting the idea of running applications right off a USB flash drive because of the idea's increasing popularity, or despite the growing demand.
There's some great directories listing all the apps designed for, or optimized for, running on a thumb drive. Jeremy Wagstaff's list is a great place to start, and if you can stomach the overwhelming amount of ads, TechTastic.ca has a good list as well.
The contrast is striking. Even the ugly TechTastic site has actual links to apps you can download today, while the U3 site has a form that lets you "Keep up with the latest U3 smart drives and smart software with our free Get Smart e-mail update."
That's not to say that U3 won't grow to be a really useful spec that's widely adopted. It just means that they're blowing the chance to demonstrate the real utility of the technology they're advocating, even in its current rudimentary and immature state.