http:://www.pixlr.com/ - This is the best website I have used lately. Today, I was able to use to make transparent PNG buttons. I loved it! Worked super fast, especially in Google Chrome.
http://www.chromeexperiments.com/ - These are some of the finest examples of what's capable in a modern browser around. Making extensive use of the HTML5 Canvas and many other techniques. Some of my favorite examples are jsCanvasBike, 100 Tweets, Canvas 3D Engine, Colors Cube.