Today we got our dishwasher fixed under warranty. To diagnose it, the tech stuck a photosensor+magnet onto the front panel over the green status LED and started reading diagnostics. It turns out the status LED is actually a serial port*, and it's continually transmitting status.
I can think of so many gadgets that could and should use this trick...
*It's probably something other than regular serial -- a UART's TX would flicker. I don't know any details.
Effectively, this is an opto-isolator. Except it's not all in one device. Wikipedia has a good article on them, and notes that they can manage 10s of KHz. (See MIDI.)
The nice thing is that there's zero extra hardware -- every gadget already has an LED or seven, and if your MCU has a spare DMA channel, you can stream a bitstream to it (however it's encoded) with low overhead. If it's a short status message, you can just let the DMA loop over it and there's no MCU overhead.
Kernel Bob 
mal3aby