This on gets lots of points for being very cool and not so many for the fact that I can't test it because it's a limited beta. This is a purely online Rails development environment. You do everything in the browser based IDE and it's "instantly" live. The screen casts are cool but spared IMHO. They note on their site that the system is "resource intensive" so I'd be really interested to load up a good app and kick out a couple of load testing runs. Is it elastic? That would be cool.

http://www.rubyinside.com/heroku-an-online-rails-development-and-app-hosting-environment-647.html