I have just started using Things, a new GTD application for the Mac and am really liking it. I saw this one tip in the forums and wanted to document it a little more clearly. Basically this shows you how to setup contexts in Things using tags without overwhelming your menu bar with a gazillion tags.
Open Tags Window
Create an @ tag
Add Nested Tags
Add nested tags for all of the contexts you want to setup.
Here I have setup four contexts, Grocery Store, Phone, Online and Email. Now assign these tags to some tasks.
Select a Focus
If there are tasks in the focus with your tags you will see the @... in the tag bar.
Click on @ tag
When you click on the @ tag you will see all of the sub items.
You can then filter the list based on the context.