MuseScore is a free cross-platform WYSIWYG music notation program that offers a cost-effective alternative to commercial & proprietary software such as Sibelius or Finale. You can print beautifully engraved sheet music or save it as PDF, MIDI or MusicXML file. Notes are entered on a "virtual note sheet" on a unlimited number of staves with keyboard, mouse, or MIDI keyboard. The score can then be played thanks to the embedded synthesizer.
MuseScore manual is available online or as a PDF file.