DJay ($50) is one of my favorite software titles here at Macworld Expo and I'm awarding it with an O'Grady's Best in Show award for it's slick interface and totally intuitive operation.
If you're a novice you'll appreciate its simple operation, djay sees your iTunes library and double clicking a track in the right pan instantly adds it to an available turntable. You can scratch a track by clicking and dragging the record and you can jump to any point in the track by clicking and dragging the turntable's tone arm. Double clicking a second track cues it up on the other deck. You can use the crossfader switch tracks or click on the transition buttons for a smooth mix. A BPM tab allows you to tap to figure out the BPM of a song. Auto-BPM would be nice, but it's not in this version.
Pros will love the multi-touch trackpad scratching (nice!), instant pre-cueing (either with USB headphones or a Griffin iMic), real-time sampler and visual loops. If you've got the "scratch," buck up for the Vestax DJ MIDI controller with dual jog wheels (about $500) and you'll be in DJ hardware heaven.