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