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.
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.
Hookmark for macOS makes it possible to link your OmniFocus projects and actions to almost anything, including files, folders, notes, websites, and emails. Having what you need within easy reach accelerates the process of getting things done.