DisplayFusion is a Windows XP and Vista utility designed for people with multiple monitors. But even single-display users should check it out.
The free app's core feature is wallpaper management. It lets you select different pictures for your desktop background on your various displays. XP by itself can't do this.
It will even let you search for Flickr photos for your backgrounds. Cool.
But what I really like about it is its built-in shortcuts for window management. There are a number of customizable ctrl-Windows commands that I've taking to using habitually. Ctrl-Win-left-arrow and ctrl-Win-right-arrow resize windows along the left and right halves of your display, respectively. It makes it easy to line up two windows side-by-side, like a browser and word processor. Multi-monitor users will also like ctrl-Win-x, which sends your active window to the other display.
DisplayFusion is donationware.