Athens Research uses a workflowy like UI. It comes with a bunch of other features, but you can ignore those pretty easily. Obsidian with the Outliner plugin is

Athens Research uses a workflowy like UI. It comes with a bunch of other features, but you can ignore those pretty easily. Obsidian with the Outliner plugin is also nice - although Obsidian isn't open source, it is free and all your data is stored locally as markdown files. 1. https://github.com/athensresearch/athens https://github.com/athensresearch/athens 2. https://github.com/vslinko/obsidian-outliner https://github.com/vslinko/obsidian-outliner