The following sites are compatible with the Amazon Kindle

www.processingjs.org
www.sketchpatch.net

Due to it's inability to open a second window these do not work
sketch.processing.org
sketchpad.cc

I was surprised to find that Studio Sketchpad is fully-functional on the Kindle, Etherpad and all. I'll take a look into the single-window limitation soon. I posted a few details and screenshots on the sketchpad.cc blog ( Code beautiful on a sunny day). Here are two: