Flex mapping gaining traction - free Flex mapping JS API
I just had a look at the Flex 1.5 based National Geographic MapMachine (recently noted by Ben Forta) which is powered by ESRI's ArcWeb Services which in a nutshell provides mapping data (we call it spatial data in our community) via web services -- they also have free data available. A number of us in this community are now starting to build Flex 2 web-based GIS applications and I expect to see more and more in this space as the UI requirements of web-based GIS applications is a perfect fit for Flex.
In the meantime, anyone wanting to have a play with stuff without getting too deep, ESRI has published the ArcWeb Explorer JavaScript API and get your own Flex mapping application going now. Note that this requires an ESRI global account -- and don't worry they don't e-mail you or anything it just gives you access to the content.