OS/2
Source: Janos Haide, Panagiotis Skagos
Make the desktop active (i.e., click on the desktop).
Press alt-shift-ctrl-o simultaneously.
You get a beach scene with a pink flamingo (and other nifty things) plus a list of the program authors.
Press any other key returns you to your regularly scheduled Workplace Shell.
OS2.04
Source: Norman St. John Polevaulter
While we're at it, the new OS2.04 has its own set of secret messages.
When workbench is running, hold down control, alt, and shift, and start selecting items from the leftmost Workbench menu. It may take a few tries to get them, but they're in there.