CheckBoxer is an extension I wrote for Google Chrome. Based on the CheckBoxMate plugin for Firefox by nrlz, it does ... well.. exactly the same thing: you draw a rectangle around the check boxes you want to toggle on or off. Magic.
Here it is in action:
As you can see, it has selected all of the check boxes within the selected area. No need to hold any other buttons as it only works when you've dragged over the starting checkbox.
Click here to check it out.