Solid state software

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):
 

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.

Last but not least a real treasure for cartridges (scanned pictures and .crt files also) can be found at mayhem64.co.uk ...