Windows only: Windows' built-in Aero Snap is pretty neat, but limited to simple left-right-full window-sizing options. Window Manager provides a customisable grid that lets you pick where and how large your desktop windows get resized.
It's similar in some ways to WinSplit Revolution, but Window Manager uses a grid layout rather than directional arrows to move windows around and size them. If you've got a large enough screen or dual monitor layout that you can imagine nine different positions for open windows, Window Manager can accommodate you. The app sits in your system tray waiting for your click; when you do click with a window selected, you get the grid plus options to centre a window.
Window Manager is a free download for Windows systems only. It requires the .NET 4.0 framework, and one other Lifehacker tester found it somewhat picky in recognising such an installation.