This little gadget could potentially be used for dubious spying activities (not real spying, obviously) and it’s not exactly the most covert device around, but the Key Shark is a keyboard logging device that will record every key pressed on an attached PS/2 computer keyboard.
It has enough capacity to store half-a-million characters so could record several months’ worth of activity.
It simply plugs in between the keyboard port and the keyboard plug, and will also work with older DIN style keyboards if you get the appropriate adaptor.
The device is mobile so can be moved from machine to machine, and itself is password protected.
Naturally its publicised uses include parental monitoring of kids (who’ll probably figure out that it’s there pretty quick and unplug it), employee monitoring (ditto), archive of data (though burning a CD might be better), and detecting unauthorised use of your PC (assuming they don’t just pinch it)
The other drawback is that many keyboards now use USB connectors, rendering this ineffective.
The keylogger costs £49.99, so you’d really want to eavesdrop on someone to get it.