I’m planning to create a cartoon animation for one of my blender projects. Since I’m a huge fan of open-source, I wondered if there is an open-source program that is inspired by flash?
The flex sdk is open source, but that’s mostly programming (not like flash).
as for tweening frames and drawing… for drawing Inkscape is probably closest to Flash… animating I can’t help you 
+1 on the request 
You might also look at Flash Develop. It can use the flex sdk and basically provides full access to Actionscript 3.0 programming. I use it along with the flartoolkit to create augmented reality stuff using Blender models.
Another option although not open source is cheaper than Adobe’s Flash is SwishMax 2 at Swishzone.com. I used it a couple of years ago and it is pretty good.
If you don’t need action script try Pencil and Synfig Studio
http://www.pencil-animation.org/
http://synfig.org/Main_Page
I tried pencil and it is a very good replacer of flash. Drawing is easy and the interface is simple. Only the export functions could be improved.
Thanks guys, you can soon expect a new thread at the traditional forum 
Found an animation package in Ubuntu, it makes .swf’s… haven’t tried it though.
Server side there is Ming.
If you tell us what it is maybe someone else can try it?
There is some handy info on using flex SDK in conjuction with papervision (and blender, of course) for 3D stuff in this thread: http://blenderartists.org/forum/showthread.php?t=163965 … might help to get going, anyway.