I'm trying to authenticate to Google Talk. The X-OAUTH2 mechanism, which uses OAuth2 tokens, is well documented. But what is X-GOOGLE-TOKEN and where is it documented?
Ah, found it. It's the deprecated ClientLogin API:
Important: ClientLogin has been officially deprecated as of April 20, 2012. It will continue to work as per our deprecation policy, but we encourage you to migrate to OAuth 2.0 as soon as possible.