http://www.ajaxsketch.com/

ajaxSketch is a web-based drawing tool that provides a familiar environment for diagramming, creating flow charts, free hand drawing and more. You can perform functions such as resize, rotate, change background or pen colors, and alter opacity...all from a Firefox browser! Once finished with your images, you can easily save your work right to your hard drive. This keeps you organized and works in the same way that you're already accustomed to.