Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Jokosher - Jokosher is a simple and powerful multi-track studio.
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.
DSP-Quattro - DSP-Quattro
LMMS - Make music with a free, cross-platform tool
GarageBand - GarageBand is a fully equipped music creation studio for Mac