To users, Pony Express is a Chrome extension that hijacks mailing list archive URLs and rewrites them into a more pleasant UI.
In pictures, a page that looks like this:
Transparently becomes a page that looks like this:
Concretely, Pony Express is comprised of three modules:
ui/. This is pretty preliminary at the moment.
pipermail/. Conceptually, other archives (like hypermail) could plug in as well.
A Chrome extension that grabs URLs, feeds them into module 2, then feeds that into module 1.