Windows

Apps

Obsidian is a highly-adaptable note-taking app. Notes are stored in Markdown text files in the location of your choice and it's free for personal use.
Craft is a meticulously-designed note-taking app for iPhone, iPad, Mac, Windows and Web. It's well integrated with OmniFocus and is a great place for reference information and project support notes, whether you're using it individually or as part of a team.
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate, and get organized.
A polished email client with a focus on email prioritization. Spark Mail also has innovative support for teams and integration with OmniFocus.
Create OmniFocus actions, action groups, and projects more quickly and accurately by defining TextExpander snippets for frequently typed words and phrases.
An email client for Mac and Windows that's speedy, customizable, and extensible. The Mac version includes native support for OmniFocus.
Evernote is a natural complement to OmniFocus. Use it to store ideas for upcoming projects and as a repository for project reference material.