Mac: One of the best elements of OS X Yosemite is the widgets in Notification Center. If you're a Spotify user, Spotify4Me stuffs a tiny little widget in Notification Center for controlling the app.
Spotify4Me gives you control over skipping, pausing and volume. That's all it does, but having those control easily accessible in the Notification Center is pretty handy. If nothing else, it also gives you a nice album artwork view to quickly pull up a look at what's currently playing.