Here are some software designed for live mixing between tracks as a DJ would on actual turntables. I have tried many of them and here are some which I think work pretty well for a DJ who wants a good combination of mobility and performance. For best results, use 320K mp3 or Wav when mixing on these software.
Traktor DJ Studio (now known simply as Traktor) is a software DJ solution developed by Native Instruments. This is actually one of my favourite and most frequently used software if I'm mixing live. I have personally used it in conjuction with a configuration of turntables and an external mixer.
The interface is straight forward and any DJ/musician would find many of its features handy and useful. It also offers a wide variety of features such as support for MIDI controller, EQs, 4 decks, multiple effects, master tempo, etc. The best part about this software is the ability to use it with Tracktor Scratch - meaning you could manipulate the digital tunes on vinyl.
Latest version available is Traktor DJ Studio 3
Download the demo from Native-Instruments here.
2. Ableton Live
Ableton Live is a powerful all-round DJ software. This piece of software is rather unique in the sense that it also allows for composing and arranging track "on-the fly". For this very reason, it is also extremely popular among many professional DJs and musicians. Some people who use Ableton in their livesets and compositions include Armin van Buuren, Sasha, Mogwai, BT, Paul van Dyk, and Daft Punk.
I seldom use Ableton in my sets, in favor of Traktor, partly due to the lower system requirements and functionality. It includes tons of features that I seldom (or never) use when mixing live sets such as track arrangement and sampler. For newcomers, the interface maybe a little daunting but the program provides a nifty little tutorial that'll guide users step-by-step.
Latest version available is Ableton Live 6
Download the demo from Ableton here.
Mixxx is an open source DJ tool designed for both professional and amateur DJs alike.
Mixxx allows DJs to mix music live with a clean, simple interface. Futhermore, Mixxx has a number of key features to help DJs in the mix: Beat estimation, parallel visual displays, and support for various DJ hardware controllers.
Mixxx is available for both Windows and Linux. The interface for Mixxx is clean and minimalist. I had only briefly tested this software but it provides basic functionality that any DJ would need for mixing (2 decks, cross-fade, EQ, queuing etc). According to the site, it also supports MIDI controllers. Best of all, it is free (as in freedom of choice and free beer) due to the nature of its GPL license.
Latest version is Mixxx 1.5
Download Mixxx from Sourceforge here.