The CP/M CARD
One of the most popular add-ons for the Apple II was a card which let the computer run the CP/M operating system. Digital Research's CP/M was designed for the Intel 8080 microprocessor and a lot of software was written for CP/M from the late 1970s to the early 1980s. The simpler CP/M cards were just a Zilog Z80 microprocessor (software compatible with the 8080) with a few logic chips which enabled it to plug into an Apple Bus slot and take over from the onboard 6502 processor.
ALS's The CP/M CARD (that's its name!) was one of the fancier ones. The Z80 on the card runs at 6 MHz, while the cheaper ones normally run at 2 or 4 MHz. It also has its own 64 KB of RAM. This means that it can use the Apple II memory for text display, buffers, I/O drivers and parts of CP/M and still have 61 KB left for running CP/M programs. Later versions of the utilities provided with the card allow the CP/M operating system and software to be installed to a virtual disk stored on a ProDOS drive. It can even boot into Apple's ProDOS, then start CP/M and continue to use the ProDOS drivers for the clock and disk drives, including RAM drives.
I picked up one of these cards cheap, along with the disks and all the documentation, at an auction (the old-fashioned kind, not online) back in 1994.
The card must be installed into either slot 4 or slot 7. The 5.25" drive controller must be in slot 6. If you want to install the software to a ProDOS drive (hard drive or 3.5" floppy) the controller for that must be in slot 5.
Apple IIGS Users: Copy the files from the CP/M Full Install archive to a 3.5" floppy disk. Put the card in slot 4 or 7. In the control panel set the appropriate slot to Your Card, the startup slot to 5 and system speed to Normal. The software won't boot properly unless you do a cold boot. To guarantee a cold boot, turn the power off, insert the 3.5" disk and turn the power on.
More info is available in the Overview and Update Notes that came with the 3.01C update.
The disks are stored in ShrinkIt 5.25" shrunk disk archives.
ProDOS disks, used for installing the CP/M software to a ProDOS drive
This is a ShrinkIt file archive containing all the files from the four CP/M disks installed to a virtual disk. If you want to run it from a hard drive or 3.5" floppy drive this is all you need. The folder must be copied to the root directory of a ProDOS drive in slot 5 and the folder name must not be changed from CPM.
The Online Software Museum has a short history of CP/M, and documentation for CP/M 2 in HTML format.