I'd second Plex, but I haven't used it for anything other than TV and Movies. I also haven't used the Audio Station from Synology. For my own music collection, I've been doing what Dan alluded to: I put it on Google Music. That way I can have an unlimited streaming music service that contains the Beatles and Led Zeppelin, something that's impossible with Spotify or rdio.

Side note: isn't Synology awesome? I'm absolutely loving mine. I have the DS412+, and I'm currently messing around with the Surveillance Station as my DVR for my nursery cameras.