Free Wallpaper Changer
I found the freeware Wallpaper Changer at one tip a day a great blog that posts a useful tip every day. The wallpaper changer changes the current wallpaper in an interval between 15 minutes and 24 hours.
It ships with a set of six wallpapers that you can switch between right away, and options to add your own ones easily to the rotation. You do not have to configure the program at all, just add some more wallpapers, delete some and set the interval, that is all.
Update: Please note that the wallpaper changer is no longer available on the website it was hosted on. But fear not, you know that we always have an alternative for you at hand so that you do not leave the site empty handed.
If you are running Windows 7 or Windows 8, you can use your operating system's personalization options to create a wallpaper pack that Windows will rotate automatically from then on.
If you are running a different version of Windows or want more control over the process, take a look at Wallpaper Master instead which comes as a free and shareware version. The freeware version is limited in regards to the file formats it supports, does not support multi-monitor systems and can only maintain up to 5000 images per category.
The majority of those limitations should not keep you from downloading and running the excellent program. It has a lot to offer, including customizable global hotkeys that allow you to quickly change the wallpaper on key press, automatic analysis of the wallpaper to make it fit as good as possible on the screen, and lots of personalization and randomizing options.
You can find more wallpapers at the following websites which are mostly spam and ad free. (no popups for instance).