Google Code Project Hosting recently adapted CodeMirror to allow for quick in-browser editing of code in a repository. This seems extremely useful! Unfortunately not all of my projects are open source.
Are there any other DVCS (Mercurial or Git) hosts that also have this feature but support closed-source projects? In particular, free would be nice (since I'd like to host personal projects with it and be able to tweak them in the browser); I'm a student, so if there's a discount or free mode due to that, that's cool too!
GitHub can do that. There is a “edit this file” button next to each code viewer.
See also the “Edit Online” section on Features / Hosting.
Bitbucket recently introduced a online code editor, just like the one GitHub provides. As Bitbucket also offers free private repositories (up to 5 collaborators) this is definitely a good alternative to GitHub.