The SproutVideo PHP SDK by SproutVideo wraps the API to access video hosting features. Developers can install with composer and GitHub. Resources present how to initiate development with videos, tokens, tags, and playlists.
The self-proclaimed movie freaks behind MovieClips now have a platform that could allow for some really fun mashups. Unfortunately, even the most basic documentation is hidden behind an application form and a promise that "access to the API is granted only to approved partners."
You can now use the YouTube API to access and create captions for your videos. That means you could let your site visitors refine the transcript to videos you own, or create their own versions of the caption.