Allegory is described as 'powerful notes app with a beautiful iOS-centric UI. The minimal and focused writing experience, combined with an impressive set of powerful features, makes this the ideal choice for everyone' and is an app in the Office & Productivity category. There are more than 100 alternatives to Allegory for a variety of platforms, including Windows, Mac, Android, Online / Web-based and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like Allegory are Microsoft OneNote (Freemium), Evernote (Freemium), Simplenote (Free, Open Source) and CherryTree (Free, Open Source).
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.
Zettlr is a supercharged markdown editor that combines many writing editor features and collect them in one application. It aims at speeding up your workflow and help you write without any interruptions.