The free AeroSnap download brings the desirable capabilities of Windows 7's AeroSnaps feature to both Vista and XP. When you drag a window to the left or right edge of the screen, the Windows 7 feature automatically resizes the window to fill that half of the screen. To "undock" a window, you just click and drag it away from the edge. This is a great perk for users who have widescreen monitors, as it lets you put a pair of windows side by side with two quick flicks of the mouse. And AeroSnap emulates another Windows 7 window treatment: When you drag one to the top edge of the screen, it maximizes.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.