Tap DJ is arguably the best of many music mixing apps now available in the iTunes App Store. It basically turns your iPhone or iPod Touch into a pair of turntables. You can access your iTunes library through this app, load up a pair of songs, and then get creative with scratching, mixing, and beat matching with a single finger.
The layout of the app is quite similar to a real turntable. You tap eject buttons to load and unload songs, tap the platter to play it, or tap and hold to scratch. There are volume and pitch controls, as well as a crossfader. A timer displays how many minutes remain in a song. There is a button at bottom of the display that pulls up an effects panel so you can inject loops and other special effects into your mixes. Tap DJ runs on iOS 4.0 and later, and even on the iPad. However, you won't find as many features as in djay for iPad.