Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
Radium - Radium is a music editor with a new type of interface.