Matt Woodward's Blog: "Laszlo Systems just announced that their RIA platform is now open source and free for deployment. I haven't checked out Laszlo in a while so I'll have to revisit this. Based on the little bit of playing around I did today I get the sense that Flex is still a more robust, capable solution, but this is GREAT news for the advancement of RIAs." (entry is old but links there are up-to-date)

Laszlo is looking very nice and it IS Open Source, more info : http://www.openlaszlo.org/