An email client that allows you to create OmniFocus actions that reference emails, manually or automatically using rules.
Mac
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.
Craft is a meticulously-designed note-taking app for iPhone, iPad, Mac, and the 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.
Daylite by Marketcircle is a feature-rich CRM and business management app designed for individuals and organizations who have standardized on Apple technologies.
DEVONthink is a powerful data storage and research app for Mac, iPhone, and iPad that's a natural complement to OmniFocus. Through the use of links, you can have a vast amount of data at your fingertips as you work on tasks in OmniFocus.
Evernote is a natural complement to OmniFocus. Use it to store ideas for upcoming projects and as a repository for project reference material.
An alternative to Apple's Calendar app for Mac, iPhone, and iPad that provides enhanced functionality, a natural language interface, and OmniFocus integration.
OmniFocus for Mac (Pro) can be automated through the use of AppleScript. FastScripts makes accessing these scripts quick and convenient.
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.
Hook for macOS makes it possible to link your OmniFocus projects and actions to almost anything, including files, folders, notes, websites, and emails.
Use Keyboard Maestro to quickly access OmniFocus features and perspectives and bring speed and convenience to frequently-performed activities.
An email client that combines Gmail's web interface with a slew of additional features, including integration with OmniFocus.
Allows you to create visual outlines of your ideas and responsibilities. Useful when designing your system and for planning projects.
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate, and get organized.
An email client for Mac and Windows that's speedy, customizable, and extensible. The Mac version includes native support for OmniFocus.
A polished email client for Mac, iPhone, iPad, and Apple Watch with a focus on email prioritization. Spark also has innovative support for teams and integration with OmniFocus.