The Control key (often labeled CTRL) may be famous for letting you Control-click on almost any interface element to reveal a contextual menu. But that workaday use is falling by the wayside now that we also have the two-finger trackpad tap and a Magic Mouse setting that provide quick access to those menus. Don't decommission the key, however: When you add it to common key combinations it lets you access all sorts of unexpected commands.
1. Open a folder in a new window (or not)
By default, a double-click on a Finder folder replaces the contents of the current window with that of the selected folder. Set your Finder Preferences (Finder -> Preferences) to Always Open Folders In A New Window, and each double-click on a folder opens a new window, cluttering even a big screen in short order. The Command key temporarily reverses your Preferences setting: If you use the Always Open Folders In A New Window option, a Command-double-click displays the folder's contents in the current window, and vice versa.
But what about us keyboard junkies? Sure, Command-O or Command-Down Arrow opens a selected folder--but "opens" is defined by that Always Open Folders preference, so it might open a new window or replace the current contents, depending on the setting. You can't add Command to override the setting because you're already using Command to trigger the shortcut. The solution: Add the Control key. Command-Control-O or Command-Control-Down arrow opens a folder while reversing your Always Open Folders preference setting.
2. Open a new window for the enclosing folder
When you're looking at the contents of a folder in the Finder, pressing Command-Up Arrow opens its enclosing, or "parent," folder. So, for instance, if you're in a subfolder in your Documents folder, you can easily move up to the Documents folder and see its other subfolders. With the Finder preference setting of Always Open Folders In A New Window checked off as the default, however, moving up in the hierarchy this way means the contents of your window changes. What if you want that subfolder window to remain open? Add Control to the keyboard command: Command-Control-Up Arrow opens a new window for the enclosing folder.
As with the basic Command-O for opening a folder, the Control key reverses the preference setting. So, if Always Open Folders In A New Window is checked, Command-Control-Up Arrow opens the parent folder in the same window.
3. Get a summary of info
Select an item in the Finder, press Command-I, and its Info window opens replete with both basic and "meta" information such as how many pages are in a PDF document. Start with two or three items selected, and you get two or three Info windows. Want the total size of all the files? Add up the numbers in the Info windows. Or, select multiple icons and press Command-Control-I for a single window, labeled Multiple Items Info; this summary window gives you the total of the files' sizes.
Using the Inspector window (Command-Option-I) for multiple files also gives the total file size, but there are some key differences between a summary window and the Inspector. There's only one Inspector window, and it's "live": Change the selection, and the information in the Inspector changes to reflect the new information. The summary window, in contrast, stays "locked" on the selection you made when you opened it.
You can select other items and open a summary window for them; the summary window for the original selection remains open and unchanged (handy when you want to compare two groups of files). As with the Inspector window, you can do some limited changes to everything in the selection, such as apply a color label or lock the items, by working in the summary window; only options that potentially apply to all the selected items appear in the window.
4. Access a restart, sleep, and shutdown shortcut
You can restart, sleep, or shut down your Mac using the commands in the Apple menu, or you can just press Control-Eject to open a Restart/Shutdown dialog box. You can then use a single key to indicate your button choice: R for Restart; S for Sleep; C (or Esc, or Command-period) for Cancel; or Return/Enter for Shut Down.
5. Jump to the Spotlight Preferences command
Your Spotlight menu is open, showing hits for your search term, but the hits don't include PDFs because you excluded them previously. You need a trip to Spotlight Preferences to change the Search Results parameters, and there's a quick way to get there. When the menu is open with anything listed in it (that is, when more than just the Search field is showing), press Control-Down Arrow to select the last item--Spotlight Preferences--and then press Return. Control-Up Arrow always selects the first item, Show All, which opens a Finder window with the search results in it.
Beware: the Control key plus arrow keys are also default shortcuts for triggering Mission Control and showing the desktop. If you've enabled these shortcuts (in Keyboard preferences), they'll take precedence over the Spotlight menu shortcuts.