BBC microcomputer hack. The beeb could run a 16K ROM located at addresses &8000-&BFFF. This was normally Basic (BBC Basic, of course!). But you could buy a special board into which several (up to 16) ROMs could be slotted (otherwise you were limited to 4). The OS could page any of the ROMs into memory, so all could supply OSCLI commands, and even take control.
This was extremely cool, and one of the reasons for the beeb's success; by comparison, the cartridge connector was merely optional, and never caught on.
Sideways ROMs available included word processors (like Acorn's View), computer languages (Forth, LISP, ISO Pascal, and even Beebug C!), utilities (several graphics ROMs), an aborted Hebrew language add-on (never finished development, but mine was the basis of my father's English-Hebrew-Greek word processing system!) and file systems and device drivers like DFS and ADFS.