iPhone: YouTube has a massive selection of music on it, but it's not easy to use Google's official YouTube app just to listen to stuff. Tuner makes it a little easier to use YouTube as your music source.
When you search for and select a track in Tuner, it automatically starts playing the music without playing the video. You can then create and save playlists, search for other tracks or search through a bunch of suggested tracks. The nice part is that you don't need a YouTube account to use these features, and I didn't run into any ads while I was listening.
It's a simple little music player without lots of features, but it's worth it for complete access to YouTube's massive library.