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 Linux and Windows
• Available in 15 languages
• GNU GPL licenced

The program has a clean user interface, with fast note editing input similar to the step-time note entry found in the popular commercial Scorewriting software packages, Finale and Sibelius.