Native Instruments Traktor Duo is one of the most widely used digital DJ software titles. Traktor Duo has a two-channel mixer that includes three-band equalization, a high-pass/low-pass filter knob, per-channel effects and cross-fade. You can supplement those two channels with up to eight additional samples. Traktor Duo will run BPM-detection and automatically synchronize any sample you bring into your session and also allows you to cut loops and set multiple cue points per track. Traktor Duo is available for Windows and OS X.
Like Traktor Duo, Ableton Live was designed with live performance in mind. As such, many of its functions work on-the-fly. Live allows you to import an unlimited number of samples and can sync them automatically using the warping function. For some samples you may need to enter the sample display dialog and manually set the first downbeat and the end-point of the sample. You can also sync samples by sliding them around the time-line in arrangement view. Ableton Live is available for Windows and OS X.
Mixxx is a free and open-source software program designed to mimic the user interface and basic functionality of Native Instruments Traktor Duo. It features automated syncing and BPM-detection of imported tracks and also supports some hardware controllers. Mixxx is available for Windows, OS X and Linux.
Harrison Mixbus is a proprietary version of Ardour, an open-source, multi-track digital audio workstation. The two programs share many of the same features, but the graphical user interface for Mixbus has a more intricate mixer. The Mixbus mixer aims to emulate Harrison's hardware mixers and has all of the same per-channel controls, including equalization, compression and limiting. Though Mixbus does not have an automatic sync function like Traktor Duo and Mixxx, you can drag tracks around in the time-line to sync them manually. Alternately, Mixbus can host a beat-matching VST plugin like Gunshopsoft xTal using FST or the DSSI-VST wrapper. Mixbus is available for OS X and Linux.