I too was using OmniFocus. But after seeing a posting from CSGiles in the Getting Things Done Public Forum, I switched to using Notes and To Do capability in Apple Mail (OS 10.5).

I have a note for each project, each w/ a brainstormed list of NAs. I activate NAs one at a time, i.e. turn the project's next NA into a To Do item. As the NA is activated, I assign it to the appropriate context calendar. When the to do is completed, I check it off. I've also got a note for Singletons.

To keep my list from having too many completed items, I deactivate NAs during clean-up. To do this, I open the project note, highlight the completed NA, and click To Do again. It's no longer a To Do. Then I highlight the next NA, click To Do, and assign it to a context calendar. Downside is that you have to manually activate NAs whereas OmniFocus automates the process. I don't mind it though.

I've got two folders, Personal and Professional, into which I file my Project notes. I also have folders: Completed and Someday/Maybe. When I complete a project, I move its note to the Completed folder. I have a note in the Someday/Maybe folder for ideas that may be singletons or projects for which I don't feel like outlining NAs yet. I also have a note in the Completed folder for completed Singletons.

Having used it now for a month, I prefer the simplicity of this system.