Audio Hero - Over 250,000 royalty free music & sound effects for videos
NtEd - Music Score Notation editor.
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!
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Premiumbeat - Premiumbeat.com provides high quality Royalty Free Music.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.