From NCH Software:
Zulu Masters Edition DJ software is a professional audio mixing and DJ software for Mac. This professional DJ software lets you mix music and other audio recordings live. When loading a music track onto a deck Zulu will automatically scan the file for a beat and assign a beat per minute (BPM), and will change the tempo in the second deck for perfect synchronization. Drag and drop music into and around the application's interface and preview an upcoming track through headphones via a secondary audio output.