Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
FL Studio - Image-Line's FL Studio, now on it's 12th version, is a well-known music production suite and the most popular beat processor on the market, due no doubt to its longevity. Read more about FL Studio.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
LMMS - Make music with a free, cross-platform tool
Radium - Radium is a music editor with a new type of interface.
GarageBand - GarageBand is a fully equipped music creation studio for Mac