Hyperdev does the cloud IDE thing pretty well; I spun up a half-serious prototype of a web app in a weekend, and sitting down at any nearby computer to hack / run / test really was a joy. I even logged in from my iPad to fix some css (not painless, do not recommend, but it worked.)
Nothing to install, no environment to configure, just log in and start editing.
I remember trying it, and Fog Creek is a company I really like and trust, but it's hard to tell if they're interested in turning HyperDev into more. I want it to be a fully professional development environment, and today it's "JSFiddle for backend".
Nothing to install, no environment to configure, just log in and start editing.