Mac OS X tip: Nothing's more heartbreaking to a keyboard lover than discovering that a common application action is lacking a keyboard shortcut. Luckily OS X makes it wildly simple to add new shortcuts for any action available in the menu bar. Here's how it works:
Open the Keyboard & Mouse preference pane in your System Preferences and click on the Keyboard Shortcuts tab. Click the little plus (+) sign, choose the application you want to assign a shortcut for from the drop-down, then enter the exact name of the menu bar item you want to execute (for example, I want a shortcut to resize an image in Preview, so I'm using "Adjust Size..."). Then just pick your keyboard shortcut, click Add, and voilà—you've successfully added a new shortcut to your Mac. When you open the app, you'll notice that shortcut is even visible next to the item so it'll be easy to remember.
This option should be available for every modern operating system, but unfortunately it's not. Windows users can still turn any action into a keyboard shortcut (specifically, check the Restrict Your Hotkey to a Specific Application section), but it requires a bit more work.