Mostly this was via the Commodore 64/128 depending where I was, we had a 128 and a friend had the 64. However a couple of times I visited somebody else who had a ZX Spectrum, and the copy protection was a bit weird.
I'd forgotten about it entirely until after hitting random on Wikipedia far too many times I stumbled across the article for Lenslok.
"Lenslok is a copy protection mechanism found in some computer games and other software on the 8bit Atari, Commodore 64, Sinclair ZX Spectrum, Sinclair QL, MSX and Amstrad CPC. The most famous game to use it was Elite for the ZX Spectrum.
The Lenslok device was essentially a row of prisms arranged vertically in a plastic holder. Before the game started, a two-letter code was displayed on the screen, but it was corrupted by being split into vertical bands which were then rearranged on screen. By viewing these bands through the Lenslok they were restored to their correct order and the code could be read and entered allowing access to the game. The device was small enough when folded flat to fit next to an audio cassette in a standard case.
In order for the Lenslok to work correctly the displayed image has to be the correct size. This meant that before each use the software needed to be calibrated to take account of the size of the display. Users found this setup particularly annoying, at least in part due to the poor instructions that were initially shipped. Additionally, the device could not be calibrated at all for very large and very small televisions, and some games shipped with mismatched Lensloks that prevented the code from being correctly descrambled. The Lenslok system was not used in later releases of Elite."
From memory the copy protection scheme for the Commodore 64 version was an asked for word that was in the manual, often duplicated with a photocopier.
As Eve Online owes more than a bit to Elite this little piece of almost forgotten computing history seems interesting.
Well at least to me.
More information > http://simonowen.com/spectrum/lenskey/