Sizer

Sizer is a lightweight utility that lets you resize any window into a number of predefined sizes.

This application installs a menu on the title bar. From there, you can change the window's aspect ratio to any of the following options: 640x480, 800x600, and 1024x768. You can also choose other sizes manually.

Also, if you want to move the window to a specific part of the screen, you can do so here. Just access the application settings by clicking on the icon on the title bar.

Sizer is an especially great application for web developers, since it lets you change a window's size with just a click. It's also useful for anyone who has to frequently take screenshots.