I've been toying for a while with writing an AJAX-based Atom client. The natural name for it would be Leukippos. (Leukippos was the teacher of Demokritos and co-originator of the Greek idea of atoms.)
Anyway, tonight I made a start. My first version of Leukippos retrieves an APP introspection document via XmlHttpRequest, parses it to retrieve the workspaces and collections and allows a user to click on a collection to retrieve it.
I'm not finished collection feed parsing yet, but once that's done and I've prettied it up a bit with CSS, I'll post it here.