Ableton Live

Description (via Wikipedia)
Ableton Live is a loop-based software music sequencer and DAW for Mac OS and Windows by Ableton. The latest major release of Live, Version 8, was released in April 2009. In contrast to many other software sequencers, Live is designed to be an instrument for live performances as well as a tool for composing and arranging. It is also used for mixing of tracks by DJs, as it offers a suite of controls for beatmatching, crossfading, and other effects used by turntablists, and was one of the first music applications to automatically beat match songs. It does not support traditional musical notation.

Official Site
http://ableton.com