I bumped into this tip totally by accident – I was recently poking around various Xcode menus when I inadvertently tapped the Option key (⌥) – much to my surprise another series of menu selections appeared.
For example, the Navigate menu in Xcode (4.6) looks as shown on the left (click the image to enlarge):
If you click the menu and hold down the Option key (⌥), the menu will look as shown on the right, where the green boxes indicate new/changed entries:
Also, depending on the menu that is active, you can also get additional options by tapping the Shift key (⇧), Control key (⌃) and the Command key (⌘).
What’s is really interesting (or embarrassing, depending on your perspective) is that modifier keys in menus are not just in Xcode, other programs also make use of these same keys. Seems in my 5+ years on a Mac I would of noticed this a long time ago, go figure.