OpenScales, which has a LGPL license will allow those who want to build “rich” (rich is the keyword for any Flash or Silverlight app, if it ain’t rich, it ain’t a plugin) web mapping applications without need to license proprietary development frameworks. You can see from the demos that this ends up being very similar to other Flex based APIs.
On top of that, it supports WMS/WFS, OpenStreetMap, IGN, KML and direct image reading. You can also deploy on mobile using ActionScript 3 (Palm Pre support), web using wlex or on the desktop using Adobe Air.
OpenScales 1.1 was just released last week so the community seems strong and working toward increasing its functionality quickly. If you are working on Adobe web technology and want an open source mapping framework, you might want to check out OpenScales.