Here's a really tiny utility for Windows XP. You can hit WindowsKey + Left or WindowsKey + Right to move and resize a window to fill the left half or right half of your screen, just like the Aero Snap feature of Windows 7.
You can also use WindowsKey+Up to maximize the current window, and WindowsKey+Down to minimize the current window.
The exe is really tiny, just 2kb in size. No system tray icon, you need to close it with the task manager.
"We are merely sprites that dance at the beck and call of our button pressing overlord."