The RedMatrix (aka "red") is a webapp like no other. It combines a multi-user content management system with an identity and permissions framework - and also includes a built-in social network and a webDAV "personal cloud". And it's all decentralised.

What does this mean exactly? It means that you decide who you want to share stuff with, and if they aren't on your list, they're not going to see it. And it doesn't matter if they have an account on the same website as you - or not. Sounds pretty simple, but it's a very hard and challenging problem to accomplish this across multiple websites. The RedMatrix makes it possible.