Watch this screencast.
Popfly takes the concept of mashups to the masses. Not only can you create your own mashups without any code, but you can also create your own blocks for others to mash up!
This is all based on silverlight, so it gives you a bit of an idea of what you can do with it.
Its also worth reading what the folks at TechCrunch say about popfly. I liked this quote:
We previously covered the launch of Yahoo Pipes and compared five different applications that let you mix data and build applications online. At the time we mentioned how this space was really heating up - and how Pipes from Yahoo simplified the creation of mashups and mini-applications by providing a drag+drop interface. Microsoft are the latest entrants in this market, and they have completely leapfrogged every other application we have seen so far
I'm looking forward to getting this one...