Actually, I'm not sure about drawing a square in one simple mouse drag, which is what I guess you want.
However, you can draw straight lines with the pencil tool:
Click on your starting point, then hold down the shift key along with the mouse button, and move to the next corner of your square.
You should see how it works fairly quickly. It tells you how long your line is in pixels as you draw, and you can see when it's straight, so drawing a square should be possible.
Hope that helps, though I agree it would be nice to have a specific tool for it.
(oh, and if you want a 10x10 pixel or 20x20 pixel square, there's preset brushes for outline squares of those sizes. Not really much help for you I guess, but you never know...)