MindNode
Allows you to create visual outlines of your ideas and responsibilities. Useful when designing your system and for planning projects.
Planning
Notion
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate, and get organized.
OmniOutliner
OmniOutliner is a natural complement to OmniFocus. Among other things, it can be used to plan projects and as a repository for project support material.