Sound card
The expansion card that gave the PC something better to say than a beep.
The original IBM PC had a speaker that could be switched on and off, and that was it. AdLib added FM synthesis in 1987 using Yamaha's OPL2, and Creative's Sound Blaster stacked digital sample playback on top in 1989 — dominating so thoroughly that games spent a decade asking for port, IRQ and DMA channel, in that order.
From the late nineties audio moved onto the motherboard: AC'97 (1997) and Intel High Definition Audio (2004) split the job between a controller in the chipset and a cheap codec chip handling the D/A and A/D conversion. A discrete sound card is now mostly a studio tool, and it usually goes by audio interface and hangs off USB or Thunderbolt rather than sitting in a slot.