You can paste this into your browser's address bar and type away:
data:text/html, <html contenteditable>
Oh hey. It works on my Kindle!
I can use this for school! Before, I just typed things in an email! ...
I just found it and thought it was pretty nifty!
NOTE: It doesn't save your draft automatically like Gmail does. (It's hard to lose your important notes if you use your email.)