music21 is described as 'Music21 is a Python-based toolkit for computer-aided musicology' and is an app in the Audio & Music category. There are more than 25 alternatives to music21 for a variety of platforms, including Windows, Mac, Linux, iPad and BSD. The best alternative is MuseScore, which is both free and Open Source. Other great apps like music21 are TuxGuitar (Free, Open Source), Guitar Pro (Paid), OpenMPT (Free, Open Source) and Rosegarden (Free, Open Source).
Aria Maestosa is an open-source (GPL) midi tracker/editor. It lets you compose, edit and play midi files with a few clicks in a user-friendly interface offering keyboard, guitar, drum and controller views.
Finale is proprietary scorewriting software developed by MakeMusic, Inc. It displays music primarily in music notation and allows for multiple tracks and very specific tweaking of instrument sounds for each track.
Dispatch re-typing of sheet music to a nuisance of the past! Let capella-scan transform your sheet music quickly into a ringing score that is ready for editing and printing. capella-scan has put an end to the laborious task of typing out notes.
alphaTab is a cross platform music notation and guitar tablature rendering library. You can use alphaTab within your own website or application to load music sheets from files like Guitar Pro or use the built in markup named alphaTex.
NoteWorthy Composer is a graphical score editor intended for the creation of sheet music, but it can also import and export MIDI and Karaoke files and can export graphical WMFs. The user interface works either from the keyboard or the mouse.
EasyABC allows you to edit and play abc files. It supports conversion from MusicXML and MIDI to ABC. The latter uses a custom implementation (midi2abc is not used) in order to produce very readable ABC output. Direct midi input is supported for people with a midi keyboard.