Taken from their website:
"Noteflight sees music education as critical to developing young minds, and exposure to musical tools helps grow our future musicians. Noteflight's approachable, natural interface appeals to both young users, and to their teachers. Because Noteflight accounts can be used from any computer with Internet access, students aren't tied to a music lab: they can learn at school, at home, at the library, and at many other places."
What is Noteflight?
"Noteflight is the best way to create, store, listen to, print, search and share notated music online. If you are a composer, you can use Noteflight to author and manage your compositions. If you are a performer, you can use Noteflight to create and share sheet music that you like to play. If you're a teacher or a student, you can use Noteflight to communicate and collaborate in learning music."
How much does Noteflight cost?
"Noteflight offers a free version to individuals for non commercial use. We also offer a premium product, Crescendo, for $49/year. Users can upgrade, or downgrade their account at anytime without losing scores. Click here for a plan comparison. "
Will Noteflight stop being free?
"Nope! Noteflight will continue to offer a basic level of service that is free. It's a fundamental aspect of our business."