A new entry into the music notation software arena, MuseScore is an open source, free music editor that sports many nice features previously found only on commercial music notation products.
MuseScore is a very flexible and full featured music editor that promises to be a replacement for the Finale and Sibelius users out there.
MuseScore isn't perfect and polished like its non-free competitors out there, but the fact that it is open source makes me feel optimistic that in the future MuseScore could be to music notation what OpenOffice now is to Microsoft Word.
It's worth taking a look for any music educator.
Below is a Youtube video I found playing Canon in D through MuseScore.
I did not work on this project but thought I would include it for all to see this software in action.