Codebox & Cloud9 share some similarities (IDE's built with web technologies).

However we have a big emphasis on modularity, ease of use. We have a Desktop app (linux/mac) coming in the pipes.

Our cloud version has offline support (so you can code even when the network cuts).

And a handful of other things. For example our hosting platform supports stacks (swappable VM images), that allow us to provide our clients with tailored environments to suit their needs (languages & frameworks they use).

Overall Codebox & Cloud9 are both interesting projects. And the Cloud9 team has definitely built some interesting stuff as well.