This is a very simple and easy tool if you dont already know it. The Curves dialog box has many neat features, I cover most of them extensively on the Photoshop DVD. Occasionally, it is necessary to correct the color of our images. A common mistake for beginning Photoshop users is to try to do this is to using the Color Balance feature. The easier and faster way is to use the Curves function, clicking on the white eye dropper (see red arrow above) and then clicking on a part of the image that should be white. You may have to click a few times here and there to get it right, but this works for me about 85% of the time. You can even tweak this by individual channel using the drop down channel menu.
Also, there is a reason Curves are so much better than Levels. Does anyone know why?