Windows only: If you're looking for a lightweight tool for compartmentalising space on your monitor and need support for multiple screens, MaxTo lets you subdivide to your heart's content. Upon first running MaxTo, each of your monitors is grayed out and a several small buttons are displayed. Using the buttons, you can divide each screen horizontally or vertically and grab the dividing bar to resize the space. Each space can then be further divided as small as you desire. The grey background is slightly transparent, which makes it a breeze to line up the dividers bars with existing window sizes (like when you've got your browser windows just so) or wallpaper details. You get five basic templates you can load on a per-monitor-basis for quickly sizing windows to common layouts without having to fuss with the settings. Once you've arrived at a happy layout and save your settings, from then on any window maximised in that portion of the grid you've created will be sized to fit.
In the screenshot above I've divided my third monitor into three panels, two for keeping track of Digsby and one as a working space for monitoring websites. If you need to see a particular window full screen hold down the SHIFT key and press the maximize button to expand it—and if SHIFT doesn't work with your current hot key setup, you can select from a list of alternatives. You can have two distinct profiles for the layout of your windows, which can also be controlled via hot key. MaxTo is freeware, Windows only. Thanks Gabe!