Solfeg.io
An app for teachers and musicians of all ages that makes it easy to practice and understand how music works by playing well-known songs together.
MakeMusic, Inc.
MakeMusic, Inc. brings together all the tools needed to teach, practice, perform, explore, and create music with MakeMusic Cloud (SmartMusic), and products like Finale and Garritan.
Musictheory.net
Fundamental music lessons, exercises, and tools you can use and share for free.