http://mixxx.sourceforge.net/index.html

Mixxx : open source DJ tool

Mixxx is designed for DJ’ing for both professional and semi-professional users. Mixxx was started in early 2001 as one of the first digital DJ systems. Today Mixxx includes many features common to digital DJ solutions, but also some very unique features: It is free, and runs on all major desktop operating systems. Futhermore Mixxx has a number of key features to help the DJ in the live mix: Beat estimation, parallel visual displays, and support for many DJ input controllers.
At the moment we are working on Mixxx 2.0 bringing some major new features, including waveform summary displays, new sound effects, and support for pitch independent time stretch.