Head over to John Montgomery's blog to read the history behind how PopFly was created. : http://blogs.msdn.com/johnmont/archive/2007/05/18/what-i-ve-been-doing-for-the-last-year.aspx
here's a quick excerpt:
In one weekend, Adam had built a prototype that laid down the groundwork for where we are today. Over the course of the next four weeks, he built an HTML-based mashup creation tool, integrated with Windows Live ID and began to demonstrate how drag-and-drop mashup creation could work.
It took another four months for us to get a team in place. In October, we reached critical mass, with enough developers and program managers to start turning the prototype into working code. By December, we could plausibly show mashup creation, storing things to Windows Live Storage (the back-end to Windows Live Spaces), sharing, and a host of other scenarios. In February, we invited in our first set of pre-alpha customers to try it out with. In March, we opened it up further and Adam rewrote the AJAX-based mashup editor in Silverlight and we introduced a host of Silverlight-based blocks. And how, in May, we're ready to take the next step and invite in a few more people and to start talking about what we're doing.
Here's a link to the Channel9 video that'll show what it does: https://channel9.msdn.com/Showpost.aspx?postid=308460
or from YouTube:
Kudos to the team, this is FANTASTIC use of Silverlight.