MuseScore is a free cross platform WYSIWYG music notation program, licenced under GNU GPL.
Some highlights:

  • WYSIWYG, notes are entered on a "virtual note sheet"
  • Unlimited number of staves
  • Up to four voices per staff
  • Easy and fast note entry with mouse, keyboard or MIDI
  • Integrated sequencer and FluidSynth software synthesizer
  • Import and export of MusicXML and Standard MIDI Files (SMF)
  • Platform independent code, binaries available for Windows, Mac and Linux
  • Available in 20 languages
  • GNU GPL licenced

The Free, Cross-Platform Sound Editor

Audacity® is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.