Shutterborg is the simplest web based word processing program that I've tried. Shutterborg does word processing and nothing more. As you'll see in the screen shot below the user interface is very clean and simple.
Applications for Education
Shutterborg is a simple tool, good for introducing young students to web based word processing. The user interface couldn't be more clean and straight-forward.
There are a couple of drawbacks to Shutterborg that might prevent it from being widely adopted. Unlike most web based word processors, Shutterborg does not allow for collaboration between users. To save work created in Shutterborg it has to be exported to your local computer as a pdf, Word, or html file.