Mopidy is a music server which can play music both from your local hard drive and from Spotify. Searches returns results from both your local hard drive and from Spotify, and you can mix tracks from both sources in your play queue. Your Spotify playlists are also available for use, though we don't support modifying them yet.
To control your music server, you can use the Ubuntu Sound Menu on the machine running Mopidy, any device on the same network which can control UPnP MediaRenderers, or any MPD client. MPD clients are available for most platforms, including Windows, Mac OS X, Linux, Android and iOS.
To get started with Mopidy, check out the docs.