"The APP Test Site is a server implementation of the Atom Publishing Protocol (AtomPub) that you can use to test your client against. It supports both entry and media collections. The server is written in Python..."
"The goal of the Apache Abdera project is to build a functionally-complete, high-performance implementation of the IETF Atom Syndication Format (RFC 4287) and Atom Publishing Protocol (in-progress) specifications."