Documentation for App URI Schemes
The windows phone developer site should have a place where app developers can search for URI schemes used in other windows phone apps, and get relevant documentation for them.
When submitting an app the store submission process should prompt you to link to documentation for any URI schemes your app includes in the manifest file, or mark them as private URIs if you don't want to document them.
This idea came from a conversation around Matt Lacey's blog post: http://mrlac.eu/1lxwDi4 where he found over 300 URI schemes from some of the top apps in the store, many of which do not have any publicly known documentation. Currently the best place to find documentation for URI schemes is the Nokia Dev Wiki http://bit.ly/NokiaUriSchemes but this information should really be available though Microsoft.
Lucian Tomuța commented
The thing is that MSFT can easily publish a list of registered URIs, but only the app developers themselves can publish the protocols built on them.
Still, it would be a good start if MSFT would publish those URIs so that those interested in using one of them may ask the developers of the app in question for details.
I know geekchamp.com also has a collection of these