Here's a piece of Windows freeware that made me go "wow" today. It's called Animosaix, and you can get it from http://www.animosaix.com/.
Anomosaix creates new mosaic images that are automatically used as your Windows wallpaper. The clever part is that each mosaic consists of one of your own photos, constructed from hundreds of your other images.
For example, if you look at the image below, the photos is of my dog Dill. And if you were to zoom in on the image, you'll see that the picture of Dill is made up from lots of my other digital photos.
Animosaix will keep creating new wallpapers, and cycle them on a schedule that you define. So you can have a new one every few minutes if you want.
And the program has another trick up its sleeve too. It creates a screensaver for you, again as a mosaic made from your photos. When the screensaver is triggered, the computer zooms and pans around the mosaic image in an animation.
You'll need at least 16 photos to get started, though more is better. The developers reckon 1000 is a good number on a reasonably powerful PC.
The program itself is a 5 MB download, is free of malware according to VirusTotal, and should run on Windows Vista and above.
One point to note. When you install it, check the licence screen that appears after you click on the setup program. There are 3 small tick-boxes at the top which offer to install some additional toolbars and other software. If you don't want these, be sure to untick the boxes before you proceed.