The raster processing chip in the Amiga. It was a specific-purpose RISC chip whose sole purpose was to generate interrupts whenever a certain scan position on the display happened. This was used for many useful purposes; for example, one could change arbitrary colors in the palette at certain positions on the scan, leading to hardware-level drawing of yummy effects (such as a colorful bar slicing through and modifying the display). Other neat effects such as changing resolution and memory banks and sprite positions were also quite simple, and led to any number of cool things.

Many demo coders on the PC went to great lengths to emulate some of the functionality of the Copper chip using a very CPU-intensive polling technique. A good example of many Copper effects emulated on the PC was the demo entitled, simply enough, "Copper" (by Surprise Productions); unfortunately, by the time PCs were fast enough to decently emulate interrupt-driven behavior through rapid polling, hardware-level hackery such as this had long fallen out of vogue, especially since it'd be more effective use of the CPU to just emulate the effect itself (rather than the functionality behind it).

It'd be funny to see the looks on peoples' faces if they were to see a textmode copper bar nowadays, though, given how rare and unknown copper effects are nowadays.