Wallpaper clocks are like good old wallpapers, except they show time, too! A wall clock is a specially designed .wcz file that refreshes your desktop every minute to keep your time precise.
1. Install an app that supports wall clocks
Chameleon Clock{Windows} Softshape.com
Powerful program with many features, including wallpaper clock support.
Dexclock{Windows} Dexpot
If you only want wall clocks, try Dexclock. It's free but shows an ad during installation.
Wallpaper Clock{Mac OS X} Jacob Bandes-Storch
Small and great freeware program!