@Marcel - A directory wouldn't be difficult to maintain if it was automatically generated. Any plugin added to the repository would simply be scanned on upload/commit, and could be tagged in a directory, noting nothing more than shortcode name & plugin from which it's from.

I don't see why this would be a privacy issue; plugins hosted on the WordPress repo are open source and browseable via Trac already, so having an auto-generated list based on them would reveal nothing that isn't already available.

If a standard way of declaring namespaces and so forth was devised, a directory for them would be useful too. (For instance, to prevent conflicts of localization namespaces, plugin option namespaces, and so forth.)