A couple of months ago, John Montgomery gave me an invite to test drive a new Microsoft project that was billed as a mashup builder. It's been on my list of "cool things at Microsoft I've been asked to check out but never investigated deeply enough" for about three months [until recently this list also included Microsoft Scout]. Thus I was quite surprised when I found out that the project was already ready for release and had been renamed Popfly.
The most succinct description of the project I've seen is Mike Arrington's writeup in his post Microsoft Launches Popfly: Mashup App Creator Built On Silverlight which stated
Microsoft will announce the private beta launch of Popfly 
this morning, a new Silverlight
application that allows users to create mashups, widgets and other
applications using a very cool and easy to use web-based graphical
interface.
...
Popfly is a big leap forward from the competitors above because it
lets you do so much more, and it is one of the nicest web application
interfaces I have ever seen. With Popfly, you can create applications,
mashups, web pages and widgets (gadgets) and it is all tied together in
a social network (as part of the Live Spaces platform) where you can
connect with other users and publishers of applications. Mashups are
created by dragging in and connecting ‘blocks’ which produce an output.
Blocks are modules that connect to various web services API’s, and even
today there are dozens of different blocks that work with a whole
variety of different web services.
See additional screen shots and a link to a screencast on the Popfly overview page here.
The application is an impressive mashup builder. It's at least on par with Yahoo! Pipes or maybe even better since it lets you mash up not just RSS feeds but also APIs from dozens of different sources including Flickr, Digg, Twitter, XBox Live and Windows Live spaces among others. Did I mention that it is also a great demo of the capabilities of Silverlight? Damn, the Microsoft developer division folks are outdoing themselves in 2007 and it isn't even June yet.
John Montgomery has a roundup of all the recent posts about Popfly in his blog post Some of the Popfly News So Far. I've been told I have three invites to the service to give out. The first three non-Microsoft people to ping me at my work email address can have them. The invitations are gone.
Kudos to the Popfly team on such a kick ass release.