Disclosure: This morning I announced that I'm working for Adobe. While I haven't officially started, consider me an Adobe employee for all intents and purposes. I have the disclosure statement at the bottom of the page, but I'll probably run this for a while just to make sure there isn't any confusion.
Microsoft announced one of the cooler Silverlight applications to be built today when they took the curtains off of Popfly. I didn't get a beta invite to Popfly, but after seeing the screencasts and reading the blog reports, I'm really excited about it. It's a psudeo-competitor to Yahoo pipes, but the user interface makes it much more accessible to the general public and makes creating mashups a lot easier.
Other than the UI, there are a few cool features. One is the PopFly Explorer for Visual Studio. With it, you can share pats of your Visual Studio projects to PopFly and share them with people. So in some ways, PopFly can act as a group code repository, which if it works the way I think it does, could make for some informative mashups. Another great thing about PopFly is that they're going to allow you to create 'blocks' for your website that make it easy for users to take your data and mash it up. It's a great way to build your brand and I like to think of it as an "uber widget" for Rich Internet Applications. Blocks currently available look like Flickr, Windows Live Spaces, Twitter and Virtual Earth. Mary Jo Foley says they have more third party tools in the works (and the twitter block may just be a generic RSS reader).
One of the most exciting things for me to see happening in the RIA world is that the companies are starting to eat their own dogfood and apply the rich solutions to their services. Popfly is one of the first examples from Microsoft, but if the response is any indication, it won't be the last. We're seeing Adobe get into this more and more with services like the Adobe Media Player and Kuler (just announced yesterday) being built with Apollo. With Microsoft having a big a services division as it does, Silverlight may see a lot of internal uses. All this is great for users!