MuseScore is a free cross-platform WYSIWYG music notation program that offers a cost-effective alternative to commercial programs such as Sibelius and Finale. You can print beautifully engraved sheet music or save it as PDF or MIDI file.

Special Features Include:
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 your keyboard, mouse, or MIDI keyboard
Integrated sequencer and FluidSynth software synthesizer
Import and export of MusicXML and Standard MIDI Files
Available for Windows, Mac and Linux
Translated in 43 languages
GNU GPL licensed

They also have loads of plugins that can be used with the software.

Visit the MuseScore website for more information and to download the software.