I myself am a musician, and I sometimes need to write sheet music as homework since I also am a music student. And occasionally I also write music of my own, but have not the economy to buy those proprietary products that is quite popular in the music world, i.e. Sibelius and Finale. So I was quite happy when I found out about MuseScore, a completely free score writing program, with some neat features in my opinion.
The features are: Multilingual (my screenshot shows swedish), you can have up to four voices per staff, lyrics and more. So before you decide to purchase the commercial, proprietary score editors you should try this out.