MozillaLabs are testing a Web Based Code Editor named Bespin! using the emerging HTML5 standard, an innovative product from MozillaLabs.

It allows you to edit and manage your webpage files and show the preview of your development! So you dont need to install a webserver on your local machine to learn building webpages.

Plus it has got load of features, one of my favorite is Collaboration. You can follow/unfollow person who can see and edit the shared webpages.
It is easily extensible with Javascript and can be used in your applications

Bespin Web Based Editor

Bespin File Explorer

Page Preview

Visit Bespin » Code in the Cloud , signup and start crunching code