CarTunes Music Player is the perfect app for the road or the gym. It lets you use gestures as music controls so you don't need to look at the device to control your music.
Upon launch, CarTunes will go through a brief tutorial showing you the main gestures to play a song, how to quickly move from track to track, and other options using a swipe-controlled interface. Simply touch to play, swipe left or right to switch tracks, or touch and hold in the middle to control a volume knob. To add songs and control the apps settings, swipe upward to open the app's menu. From here you can create and edit a playlist using songs from your music library, change what information each track shows on screen, and switch the font to your liking. You also get a menu that shows you all of your available gestures, with some options that might make it easier for you to use (you can switch volume control to a single vertical swipe, for example, if that's easier than using the volume knob). You can assign other functions to more complex gestures (a three-finger tap, for example) as well, making it easy to set the app up just the way you like it.
Overall, CarTunes Music Player is an excellent idea that makes it easy to control your music without looking at your iPhone--perfect for when you're on a road trip or exercising and don't have time to fiddle with controls.