Bear
Bear is a beautifully-crafted note-taking and writing app for iPhone, iPad, and Mac. It can be a handy place to keep notes about your OmniFocus actions and projects and other reference information.
Airmail
An email client that allows you to create OmniFocus actions that reference emails, manually or automatically using rules.
Evernote
Evernote is a natural complement to OmniFocus. Use it to store ideas for upcoming projects and as a repository for project reference material.
Fantastical
An alternative to Apple's Calendar app for Mac and iOS that provides enhanced functionality, a natural language interface, and OmniFocus integration (Mac).
FastScripts
OmniFocus for Mac (Pro) can be automated through the use of AppleScript. FastScripts makes accessing these scripts quick and convenient.
Hazel
Using Hazel for Mac and a little AppleScript, actions can be automatically created in OmniFocus for Mac (Pro) as files are added to specific folders.
Keyboard Maestro
Use Keyboard Maestro to quickly access OmniFocus features and perspectives through global hot keys and to tap into OmniFocus' AppleScript support.
Notion
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate and get organized.
Mailplane
An email client that combines Gmail's web interface with a slew of additional features, including integration with OmniFocus.
MindNode
Allows you to create visual outlines of your ideas and responsibilities. Useful when designing your system and planning projects.
Postbox
An email client for Mac and Windows that is speedy, customizable and extensible. The Mac version includes native support for OmniFocus.
Spark
A polished email client for Mac, iPhone, iPad, and Apple Watch with a focus on email prioritization and support for teams.
TextExpander
Create OmniFocus actions, groups and projects more quickly by defining TextExpander snippets for frequently typed words and phrases.