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.
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, projects and reference information.
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.
