If you run Linux and use MPD, MPDroid is an awesome app that allows you to remotely control your computer and music library from your phone. You can browse, set up playlists, turn up/down volume, etc.
I also recommend F-Droid. Lots of FOSS programs to choose from in its own repository program separate of Google Play if that's your thing.
Syncthing is a great cloud storage app that's complete open source. You set it up on your computer, point it to a directory, and then share that directory with your other devices. It works great, and you don't have to worry about data caps and sizes.