I've been using EnvKey in production for over a month and find it indispensable. It's completely solved the nagging problem of devs of my team adding or altering ENV vars in our 4+ Ruby apps without alerting the rest of the team, leading to inevitable false app starts or bad deploys. It's also simply a cleaner and more secure way of storing this data and editing on the fly.
It seems that a ton of thought has been put into security so that it is usable on the server. I also like the ruby, node, and command line libraries which are easily installable. The api key interface feels fairly intuitive.
Good to see innovation in this space, it's a tough problem to improve on.