The YM2151, aka OPM (FM Operator Type-M) is an eight-channel four-operator sound chip. It was Yamaha's first single-chip FM synthesis implementation, created originally for the Yamaha DX series of keyboards.
It was also used in arcade games, starting with Atari's Marble Madness, and later being licensed for use by many other companies including Sega, Konami, Capcom, Data East Pinball and Namco, with its heaviest use in the late 1980s, as well as in Sharp X1 and Sharp X68000 home computers.
This chip was used in some Yamaha budget electric pianos, such as the YPR-7/8/9.
Finally, the chip was used in the Yamaha SFG-01 and SFG-05 FM Sound Synthesizer Units. These are expansion units for MSX computers and were already built into some Yamaha MSX machines like the CX5-M. It provides the FM synthesizer with stereo output jacks, MIDI ports and a connector for an external keyboard. Note that at least some SFG-05 modules contain a YM2164 chip.
It was paired with either a YM3012 stereo DAC or YM3014 monophonic DAC.
It has also recently been included in the upcoming Atari 7800 expansion module to extend the capabilities of the Atari 7800 console to include Arcade sounds.