Just read this on the blog of Somasegar:
Popfly is a tool and social network that provides a fun and easy way to build and share mash-ups, gadgets, Silverlight experiences, and Web pages using pre-built “Blocks” that connect to online services.
I asked for an invitation as they're not giving much away at the moment. Funny thing when I visited the site was to see someone I knew: Steven Wilssens, a Belgian developer and MVP that went working for Microsoft since last year.
Update: I just found out about this video on Channel9: PopFly