Not exactly DJing, but I do a lot of remixing. Nothing beats Ableton Live, in my humble opinion. From reading Ableton's forums, it seems a lot of people also use it to DJ.
I use Native Instruments Traktor 2.5 on my tiBook with an m-Audio 4-channel firewire soundcard, an external mixer and and an m-Audio O2 Midi keyboard/controller...
you can assign almost everything to midi and so I'm able to do stuff pretty fast with the O2 and a mouse...
I also have a third channel on my mixer that I use to plug my iPod or an MPC 2000xl into...
I doubt that there is better and more developed DJ-software out there than Traktor (except Traktor FS which you control through vinyl, but that's way out of my league...)
you can download a demo version of Traktor 2.5 on NI's site I posted on top...