Mixxx is a very nice and free software for the laptop DJ which runs on three platforms: Windows, Mac and Linux. And that's not the full story: it is developed with a real open source effort, and they did almost all the important functions we got used to in the expensive packages: plays mp3, OGG, WAV, FLAC, has real time stretch and BPM detection, supports numerous hardware controllers natively, supports multi-channel sound devices, has multiple skins, uses on multi-core CPUs, the maximal audio resolution is 24/96, crossfader's envelope is selectable - just to name some. Get it here.