MuseScore is a leading software to create, play and print sheet music. It is free and open source music notation software that runs on Windows, Mac and Linux. MuseScore is easy to use and it provides a way of sharing scores online via MuseScore.com. You can download MuseScore for free from http://musescore.org
There will always be a free version of MuseScore.com. It will be limited in some ways, but you can get more by upgrading your free account to Pro Account.
No. The MuseScore notation software is needed to create your own scores or edit ones you find online, but you can view, print, and play scores directly on MuseScore.com with no special software required.