The app is free, but a $1.99 in-app purchase lets you use TodayRemote with Spotify, Rdio, VLC and Vox, while also adding global volume control and the ability to control multiple Macs from a single iPhone. TodayRemote lets you add a widget to Notification Center, giving you access to playback and volume controls just by swiping down from the top edge of your iPhone. With the TodayRemote app, you can control iTunes on your Mac with the added benefit of not needing to open an app on your iPhone to do so.
Apple has long offered the Remote app that lets you use your iPhone to control Apple TV or iTunes on your Mac.