Mixxx is an open-source DJ mixing program for Windows, Mac OSX and Linux. It allows anyone with a computer to mix without spending any money. The 1.7.0 update brings customizable Midi scripting, a slew of bug fixes, and support for a wider range of Midi controllers.

While I have not tried Mixxx myself, it has a set of features to rival commercial software. Some of the notable features include:

• Vinyl Control support for timecode by Serato & Traktor
• MP3, OGG, Flac, and Wave playback
• Waveform summaries and Parallel waveform displays
• In-Software mix recording
• Multiple soundcard support

Visit Mixxx.org for more information and download version 1.7.0. Also be sure to check out version 1.8.0, which includes looping and better beat detection, when it is released in ~6 months.