NtEd - Music Score Notation editor.
Music For Makers - Licensing good music has never been this easy.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
TunePocket - Download 11,000+ royalty free music tracks and sound FX. Buy once, use forever. New music added daily. Save over 50% compared to most popular similar services. Incredible value!
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Incompetech - Royalty Free and Creative Commons Music Archive