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/staff
• easy and fast note entry with mouse, keyboard or midi
• integrated sequencer and FluidSynth software synthesizer
• import and export of MusicXml and standard midifiles (SMF)
• platform independent code, binaries available for Linux and Windows
• GNU GPL licenced