Plug and play does not exist since PCs have easy to install equipment (like USB devices). A lot of years earlier software cartridges were offered for several computers, one of the first ones for the Commodore VC-20, but in a larger number also for the C64.
Game cartridges (also called modules) were most popular, but there were also a few application modules like 'magic desk'.
The boards with ROMs were different, not just only one type was produced:
Magic Desk 1 (4x 8KB):
J1 and J3 shorted allows 2364 ROMs
J2 and J4 shorted allows 27xxx ROMs
Avenger (an Invaders clone), 8KB:
International Soccer (2x 8KB ROM):
The below mentioned COMAL80 cartridge:
If you plan to build your own (EP)ROM-cartridge, you will find still some empty boards at eBay, but you can build them from scratch also:
Just read this german article here, page1 and page2 - if you want to create the PCB, you may have to resize the picture of the PCB (you have to measure a socket with 24/28 pins and then you get the scale for it).
If you want an almost complete list of cartridges, download the list here as RTF.
A screenshot from a running COMAL 80 cartridge. The memory size of the cartridge itself is 64KB:
Last but not least a real treasure for cartridges (scanned pictures and .crt files also) can be found at mayhem64.co.uk ...
Btw. ... here is a really rare and unique software - a Fortran Compiler for the C64 (without using the Z80 cartridge!). Abacus created it in 1986 and it looks like this:
It works by masking the BASIC ROM out (by Poking an address) so almost all RAM can be used.
The source files are SEQ files on disk, but I did't manage to compile anything (there is a DOC-READER.FOR.PRG to view files, this does not working also).
>This< file contains the list of the files on disk.