Are there any extensions, plugins or addins available for the Zune software?
Do you mean the client software, or the device software? For the device, you can develop games using the XNA framework. Those "games" can interact with most of the content on the device (except DRM protected content).
It appears to now get the info directly from the Zune software. Previously it read the info from zune.net RSS feeds.
Source code is available.
There is an API for the Zune software, but as of yet it's undocumented. See Tom Fuller's SOA pitstop article.