Cyclone Wallpaper Changer
Full user review
Works great--I've got probably 150 wallpapers in a folder and it uses them to change my wallpaper background every 15 minutes (you can set your own interval time). Works perfectly!
None at all. This was exactly what I hoped it would be.
I am so satisfied with this software you might think I'm a hack for the company that makes it, but I'm not. It really is that good.