One of my [most viewed blogs][1] is one of the most simple demo apps I've built. It's a simple Flex app that lets the user draw with their mouse and then lets them save their drawing. I've updated the Flex Paint demo to Flex 4. Check it out:
<iframe src="/demos/FlexPaint4/FlexPaint4.html" width="440" height="440" style="border:0px"></iframe>

([View the source code][2])

 [1]: http://www.jamesward.com/2009/04/16/flex-paint-2/
 [2]: /demos/FlexPaint4/srcview/index.html