Joel Spolsky just announced a new service called HyperDev. From the announcement:
HyperDev is going to be the fastest way to bang out code and get it running on the internet.
Currently, the servers are Node-based but they plan to add other ones in the future.
You can read the full announcement here and then check out HyperDev. This looks like it’s going to be a pretty cool service.
That actually seems pretty cool. I’m intrigued by the simultaneous multi-user editing that works better than you might expect 
Holy crap. JSBin on Steroids!
That hot re-loading of both front and back-end is freaking amazing!
I wonder at which point it starts slowing down, but as Joel mentions in his announcement post: It’s perfect for small mini-projects to quickly prototype an idea and have it instantly hosted and shared.
I haven’t tried it out, but apparently you can also use live collaboration (two people messing with the same code file at the same time… what the actual f**k?
So is it like http://codepen.io/ but better? Meaning that I can use it like sandbox for developing web-apps without any local editor, server etc.?
More or less, as long as you’re looking for a Node-based environment. They will be adding additional development environments in the future. The real strong point (other than being ready to use immediately without setup/deployment/etc) is the real-time collaboration.