Windows only: Changing up your desktop wallpaper can be really fun, but if you want a certain image on your desktop at all times, Overlap Wallpaper takes any image on your computer and displays it over your desktop.
There are any number of things you might want to keep on your desktop, whether it be something aesthetic, like a picture of your family, or something more useful, like your daily schedule. Overlap Wallpaper is a neat utility that lets you display an image on top of your wallpaper, giving you the ability to keep that important image close without giving up your giant stash of rotating NASA wallpapers. What you put on that image is up to you, and can serve any number of uses.
You could display daily reminders, list important phone numbers and other quick-access info, or even show a list of your long-term goals to help keep your mind on track. I'm currently using it to display a small version of the Lifehacker daily schedule so I stay on top of my deadlines.
If there's one complaint I have, it's that it display's the image over everything on your desktop; that is, any desktop icons will fall under it instead of on top. If you could put icons on top of it, it'd be a great way to keep your desktop organised without giving up the wallpapers you love. Overall, though, it's a cool program, that you can use to keep yourself on task or just to add a bit more spice to your desktop.
Overlap Wallpaper is a free download for Windows only.