We now have a real-time, interactive editing environment on Khan Academy for JS/ProcessingJS, HTML, and SQL. Last year, we decided to make the editing environment open source on Github, to share how it works and encourage others to contribute. We've since had multiple contributors, and the most prolific, Kevin Barbarash, joined KA as a full-time employee.
If you're curious how it works, you can watch this talk from John Resig and this talk from me (Pamela Fox). You can also read this deep dive into the JS editor or browse through the code itself. If you're willing and able, we'd love more help in tackling the many complex issues that we run into when creating this cross-browser, real-time environment that's used by millions of students.