It is powered by the Ajax.org platform + makes use of HTML5, node.js, socket.io and many other projects.
The application has a pretty fast text editor with bundled syntax highlighting support for JS, HTML, CSS and mixed modes.
Cloud9 IDE has integrated debuggers for node.js and Google Chrome which can be started, paused and stopped from the IDE.
Using WebDAV, it can connect to remote workspaces too and has a plugin system for improving it further (already has several bundled plugins).
P.S. The application is still in early development stages, can be a little buggy but improved continiously.