On the somewhat related note of system-level things that can be done in web browsers... here is a project that I've been working on for several years now that is meant to be an entire desktop userland: https://yc-prototype.appspot.com

(I just submitted it to YC in the recent application batch)

There is a Terminal application with a fairly complete shell implementation.

I am currently working on integrating the *nix command line environment by bringing in PNaCl implementations of vim and python. So far, I have vim working as normal in my local development system, and python integration is underway. I should be getting a decent numpy/matplotlib developer environment up and running in the coming weeks.

It is Chrome-only, due to the dependency on the HTML5 Filesystem API and PNaCl.

ps. If you are trying to use the Terminal and it doesn't seem to respond to key presses, a simple refresh of the page should do the trick. There is a weird bug that I have yet to start tracking down.