[toomuchtodo]

There is no such thing as an open source CDN; there is a free CDN called Coral Cache (http://www.coralcdn.org/). CDNs cost money because you're dragging content from an origin to edge locations all across the world, and keeping it hot for client requests.

You could simply serve the content out of nginx, but you wouldn't see the performance benefits of keeping your content closest to the end user.