Very useful not-very-known selection tool.

http://mediawiki.blender.org/index.php/BlenderDev/SharpFlatSelect

While looking at this wikipage, I found out that it was already in Blender. Press Ctrl+Shift+Alt+F in face selection mode for Min Angle selection, or Ctrl+Shift+Alt+S for Max Angle selection(doesn’t work in face selection mode).

Um, you can pretty much find almost the exact same thing in the select menu when you’re in edit mode, for face mode you can find linked flat faces in the select menu.