Adobe AIR is proprietary. You could also use JavaFX or plain old Java. It's under a GPLv2 or CDDL license.

(Some people care about free software, others don't. Just thought I'd mention an alternative)

Actually, the flash platform is largely open source. The AIR framework (along with the rest of Flex) is open source. See http://opensource.adobe.com/wiki/display/flexsdk/Get+Source+...

The AIR compiler is also open source. The only thing that isn't open source in the stack is the flash player runtime, which executes the resulting SWF file. However, the SWF file format is open.

If the VM - the most critical part of the platform - is proprietary, then the platform isn't largely Open Source.

http://titaniumapp.com/ if you want an open source AIR equivalent. It's a work in progress but seems promising.

also http://cappuccino.org/

Er, that's JavaScript and only runs in the browser. The toolkits we're discussing are concerned with desktop apps.

Well the parent mentioned extjs, which is javascript and only runs in the browser.

And you can run both (last I checked) in AIR as AIR contains WebKit.

Great if you want an application to look like a web app, but it's not an ideal solution by any stretch.