How do you quickly lock your computer screen on the Mac?
Yes, you may use iAlertU, but sometimes I simply want to lock and not arm my Mac.
You can lock quickly by using the Dashboard and Exposé settings. Press Alt + F4 (or fn + Alt + F4) and type dash in the search window and hit return. This will bring up the Dashboard and Exposé System Preferences dialog. Under Active Screen Saver, select one of the four corners and set it to "Start Screen Saver." For instance, I've chosen the top left corner. Now, close the dialog. Anytime you run the mouse to top left corner, the screen saver should kick in.
You can also configure this under the Screen Saver preferences by pressing the Hot Corners button.
The above steps only started the screen saver. In order to make this really a lock, you need to turn on one more setting (which you may have done already). Under Security System Preferences (Alt + F4 and type Security and hit return) turn on the check box "Required password to wake this computer from sleep or screen saver."