Free & Open Source Software is exactly that.

You download the source code, which you are free to modify and redistribute.

Linux is a free operating system.

OpenOffice is a free productivity suite.

Eclipse is a free IDE.

Music applications are lagging behind theie commercial cousins somewhat, but there are some signs of progress...

Ardour is a fully fledged multi-channel DAW, along similar lines to Pro Tools.

Rosegarden is a MIDI sequencer, like early versions of Cubase or Cakewalk.

Audacity is a sample editor.

LADSPA is a plugin architecture like VST.

Jack is an audio routing subsystem, supported by most Linux / UNIX audio apps, which allows
you to route audio from one application to another and so forth. As far as I am aware there is no Windows equivalent for
this.

PureData is a modular DSP environment like Max/MSP or Reaktor.

Basically, I like the idea of total freedom from commercial software in making music.