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, Android and iPhone. 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).
With TuxGuitar, you will be able to compose music using the following features Tablature editor Score Viewer Multitrack display Autoscroll while playing Note duration management Various effects (bend, slide, vibrato, hammer-on/pull-off) ...
Guitar Pro is first and foremost a program designed to edit tablatures for guitar, bass, and other fretted instruments from 4 to 8 strings. Its great ease of use and the success of its specific file format have made it a program used by guitarists worldwide.
GNU LilyPond is a computer program for music engraving. One of LilyPonds major goals is to produce scores that are engraved with traditional layout rules, reflecting the era when scores were engraved by hand.
Impro-Visor (short for “Improvisation Advisor”) is a music notation program designed to help jazz musicians compose and hear solos similar to ones that might be improvised. The objective is to improve understanding of solo construction and tune chord changes.