For those who like to frequently switch their desktop wallpaper, John's Background Switcher (JBS - Freeware 1.25MB) is a must have utility. You can have your desktop wallpaper change as often (and frequently) as you want.

From the developer's website:

John’s Background Switcher (or JBS for short) periodically changes the background image on your computer (like every hour or every day) to something interesting. You can specify which pictures to choose from:

• Individual pictures on your computer
• Folders containing pictures on your computer such as ‘My Pictures’
• Flickr photo sharing - selecting pictures by person, tags, sets or just plain random. You never know what you’re going to get next!
• Phanfare web albums - keep up to date with your friends and family
• smugmug photo galleries - yours, your friends, anybody’s!
• Picasa Web Albums - choose from specific albums or any search text
• Yahoo! image search - the internet is your oyster!

Download it and give it a shot.