I've been following Popfly for a long time. It's come a long way (and 2 code names) to where it is today and it's good stuff! They have a cool editor in Silverlight for connecting blocks to create mash-ups, a web page editor, social networking to share your creations. If you're into the nitty gritty you can even go in and edit all the Javascript yourself. Check out the intro video on Channel 9.