The most ubiquitous card is probably the Apple Supper Serial Card (SSC). It is hard to find an Apple II without one. An SSC has to be configured with DIP (Dual Inline Package) switches! Like many cards, this one takes advantage of the clever memory mapping of Steve Wozniak. The programming in the ROM can be started and the ROM will discover what slot it has been plugged into. The ROM code is automatically accessed in some memory space reserved for cards in slots.
With the addition of the expansion slot, the 80 column card became essential. Once you had seen 80 column text, the Apple II native 40 column was hard to go back to.
Many printers and other devices needed a parallel interface. This is a late model very small PIO card.
To compete with third party sources and meet demand for more memory in the //e and //gs, Apple produced this very nice 1 Mbyte RAM card that fit a regular slot and was often used as a RAM disk for fast access to data.
As mice became popular, and ProDOS and paint applications became available for the //e and //gs, Apple produced this mouse and mouse interface card. It works very well.
Apple built a 5 and 10 Mbyte hard drive called the ProFile. They were so robust that Apple's programmers carried them from place to place or to home and back like a briefcase. I believe the ProFile was first intended as a Lisa product but was adapted to the Apple II with the interface card below. An Apple II boots in about a second from a ProFile. Very Impressive. This unit was used internally at Apple and has a 5 Mbyte PROM and a 10 Mbyte Profile drive. (Anyone with a card set up for 10 who can copy the PROM please contact me).
Here is the drive. It is as wide or a little wider than an Apple II and rather heavy.
A view of the ID plate. Note the serial number :-)