Indigrid is described as 'A new medium for your ideas—augment your thinking with a text editor designed for structuring lists. Side-by-side views allow 3 modes of thinking' and is an app in the office & productivity category. There are more than 100 alternatives to Indigrid for a variety of platforms, including Mac, Windows, Linux, iPhone and iPad. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Microsoft OneNote or Evernote. Other great apps like Indigrid are Obsidian, Notion, Simplenote and CherryTree.
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.