Evernote
Evernote is a natural complement to OmniFocus. Use it to store ideas for upcoming projects and as a repository for project reference material.
IFTTT
IFTTT (If This Then That) is a free web-based service that can automatically trigger the creation of actions in OmniFocus based on a wide range of conditions.
Notion
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate and get organized.
SaneBox
A web-based service that helps you to tame your email and that reduces the need to create and manage Waiting items in OmniFocus.
TaskClone
Use Evernote to take notes and to capture action items. Then add a tag to the note and TaskClone will automatically create an OmniFocus action for each of your action items.