MuseScore is a free WYSIWYG music score typesetter for Linux, 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 for Linux, Windows and Mac
- GNU GPL licenced
This app is fun,I recommend to try,whatever you know music or or not.