PC speaker, also referred to as a buzzer or beeper, is a term used in computers to refer to a simple speaker attached to a timing chip on a computer to provide diagnostic information during the booting process and basic audio feedback to the user. The first mass-produced computer with a built-in PC Speaker was the Commodore PET, and most computers throughout the 1980s were equipped with a simple 2.5" diameter magnetic speaker. It wasn't long before game developers began to push more complicated sounds through the speaker including basic music and sound effects, and, even digital sound effects and identifiable speech. As sound cards began increasing in popularity, computer manufacturers began to rely less on the speakers and they decreased in size to a 1" magnetic speaker, and now, they're often a 1 cm piezoelectric tweeter or a moving iron speaker. As hobbyists have come to understand the capabilities of the PC speaker further, they have even been able to simulate multi-voice music on them.
My family's first computer didn't have a sound card, so I was stuck listening to PC speaker beeps for much of my PC-based video game childhood. Also, since I programmed a lot in QuickBASIC, which didn't have any commands for accessing sound cards, I was limited to the PC speaker for music and sound effects in my programs (commands included BEEP, SOUND, and PLAY as well as a couple ways to buffer PLAY). Because of this, I was really blown away when I first played Last Half of Darkness which plays human speech through the PC speaker, and, then, later by demos like the ending of the 8088 MPH demo. I'm still floored by those who are able to write impressive music drivers for the PC speaker.
The following are games which I think make good used of the PC Speaker:
A 2.5" magnetic speaker on an IBM 5150.