I've just started using this powerful tool. It also happens that it can be self-hosted which I am huge advocate for in any web software. I generally use it to tinker around with HTML and CSS as I'm not that good with Javascript but I could definitely see it being useful when I learn more about that language.

Now I just need to keep looking for a self-hosted solution that allows for real-time collaborative code sharing. And not a full on IDE, just something with nice syntax highlighting for multiple languages that lets me work on code in real time with a friend or co-worker.

My JSBin Instance.