Menu Block: Because Primary Links Sometimes Need to Have a Second or Third Level.
File this little gem under damn useful.
The author pitched menu blocks better than I could:
So if you’re only using your theme’s Primary links feature, you can add and configure a “Primary links (levels 2+)” block. That block would appear once you were on one of the Primary links’ pages and would show the menu tree for the 2nd level (and deeper) of your Primary links menu and would expand as you traversed down the tree... Pretty simple, eh? (I’m actually shocked this module didn’t exist before.)