Do you want to quickly see two windows side-by-side on your desktop? Yes, you can manually resize them by dragging and dropping the edges of each window, but there is a much faster way.
In Windows 7, you can easily arrange two windows next to each other by dragging them to the left or right side of your screen. Left-click on the taskbar (the bar across the top of each window) and drag and drop the window to the left or right side. Release when the entire window is highlighted.
In Windows XP or Vista, hold the Ctrl key down and click on the taskbar of each of the windows, then right-click the taskbar in one of those windows and select Tile Vertically. In XP and Vista you can use trick to arrange more than two windows.
Nice to see multiple windows at once. And if you want to take it to the next level, get yourself two monitors.