Sometimes you just need something that is ridiculously simple, and don't want a cobbled mess in your cabinet. This is a two-channel continuity tester. The customer needed to apply 20ma through some conductive rubber bumpers, and their existing method was shocking the machine operators.
It mounts on a DIN rail and completely isolates the user from machine power. About a week from a sketch to installation.
"John, our machine just sent me a fax....."
This is my all-time favorite. It connected to a Modicon PLC, and when the PLC found trouble in the system, it would send preformatted faxes to haulers, repairmen, and managers. It also dialed up user's text pagers and delivered trouble and status messages. We a problem occured, the unit took over the modem connected to the PLC, did its stuff, then went back to sleep.
Stuffed into that 9" x 3" box is a 386 computer and an interface board built here for the job. It's old hat now, but in 1996 it was hot!