If you design for the web, you know how important it is to design for specific window sizes. To make this a lot easier, there's an application called Sizer that lets you resize any window on your computer to an exact size that you specify (no more grabbing the sizing handle and try to add or remove that 1 pixel). This application can also be really useful when creating screenshots for documentation since you can then ensure that all of your screenshots are from windows of the same size.

You can read more about Sizer and download it here: http://www.brianapps.net/sizer.html