MuseScore.org is cross-platform, multi-lingual, open-source music notation software that allows you to create, playback, and print sheet music. It features an easy-to-use WYSIWYG editor with audio score playback.
Guitar Pro is a tablature editor software for guitar, bass, and other fretted instruments. Equipped with a powerful audio engine, it makes writing music easier, and it constitutes an essential tool for guitarists.
MuseScore.com is a community site to share sheet music with social features such as embedding, commenting, annotating, statistics, and groups.