These were the two programs I constantly recommended to dual-display lovers.
DisplayFusion
It comes in a free and a Pro version: wallpaper management, timers, and — most importantly — a different wallpaper for each monitor, plus hotkey extras. It can even load pictures from Flickr automatically. Still actively developed by Binary Fortress Software — displayfusion.com.
UltraMon
My favorite back then: manage wallpapers, display settings for each monitor, screensaver settings, and a very stable multi-monitor taskbar. The nicest touch was an added button in every window’s title bar that moved the window to the next monitor — even when maximized.
An update, nearly two decades later: Realtime Soft stopped releasing UltraMon after version 3.4.1 in October 2018, so it still runs but hasn’t kept pace with modern Windows. If you’re setting up multiple monitors today, DisplayFusion is the safer bet — and Windows itself now covers the basics, with per-monitor wallpapers, virtual desktops and Win+Shift+Arrow to fling windows between screens.