Crossnote is described as 'is probably the world's first markdown notes reader & editor Progressive Web Application that works offline and supports syncing with arbitrary git repository right inside your browser' and is an app in the Office & Productivity category. There are more than 100 alternatives to Crossnote for a variety of platforms, including Windows, Mac, Online / Web-based, Linux and Android. The best alternative is Joplin, which is both free and Open Source. Other great apps like Crossnote are Obsidian (Free Personal), Standard Notes (Freemium, Open Source), Google Keep (Free) and Microsoft OneNote (Freemium).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.