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.