Popfly is a web-based tool that lets you build a new site, application, gadget or even just "pimp" your site, very very easily. It helps you build these applications without really having to deal with code. The Popfly experience is brilliant - you can drag and drop "blocks" on to your workspace, which will form the core of your site. For example, d-n-d a 'Flickr' block on to your site, give it some parameters like, oh, I don't know some tags, and pass the output of the block on to say the 'Carousel' block which shows the images extracted from Flickr (with the parameters you specified) in a carousel.
Imagine doing that in code - how long would that take you? That, up there, took me, like, 30 seconds. Seriously.
Popfly comes with a certain number of blocks, but you're not limited to just those (you can see the blocks in the left pane on the image on the left above). You can build your own blocks. And you can share those blocks with others. The user interface itself is just mind-blowing. The first question I had for Dan when he showed us Popfly was, when is Visual Studio going to look like that? (the answer was, it's not :|) It has such an awesome and intuitive interface for building apps. Seriously.
Popfly is going to have an incredibly viral element to it. Not only can you build sites, but you can then share that site with your Popfly friends. Your friends can then 'rip' your site or whatever it is that teenagers do today.
You're probably thinking, what about projects I create using Visual Studio? How can I upload them to Popfly? Use the Popfly explorer!
So, all said and done, here's the awesomest thing about Popfly, the big bottom-line, what really thrills me about Popfly - guess what, it's built on Silverlight. That's right. I just got goose bumps.