Microsoft PopFly is a cool Silverlight based mashup site. I am not exactly sure what else to say about it other then it has a Duck as a logo!
Its pretty cool, and allows for easy development of Mashups. I say development, its fairly straight forward. Eric Griffin has some more info about it here. Well, not only that, but there is an SDK for developing the blocks (again, thanks for Eric!). This is very cool!
[posted in the future using Windows Live Writer]