Atom Protocol Exerciser
Tim Bray has released Ape, a tool that will check an Atom Publishing Protocol implementation for conformance (somewhat, but not entirely similar to the Feed Validator).
This is a single archived entry from Stefan Tilkov’s blog. For more up-to-date content, check out my author page at INNOQ, which has more information about me and also contains a list of published talks, podcasts, and articles. Or you can check out the full archive.