IBM's braindead decision to have 5 (instead of 8)
expansion slots meant
 that aftermarket "multifunction" cards became quite popular.  MANY of
 them advertised SIX functions; after memory, serial, parallel, and
 joystick, how many people remember what purported to be the other two
 functions? 
Real time clock.
I don't recall whether the sixth function was a floppy controller or
a second serial port.