Depends on your budget? This is what I did,
I purchased an Edirol Optical I/O and RCA I/O to USB adapter, plugs straight into OSX. The beauty about this is that with optical you cannot spike the computer with power amps or mixers. The RCA means that you can still utilize standard leads, as this is the most used DJ connection. There is a plain RCA to USB connector as well for a little less $.
The software I use for this is SoundEdit16 and Peak DV. I know its not DJ software but the art is in mixing on the turntables not on screen.
WARNING_using 3.5mm straight into the computer you can run the risk of spiking your computer as sometimes mixers and amps have slight earthing problems. This happened many times to me.