The YM3526, better known as OPL (for "FM Operator Type L"), is a sound chip that was manufactured by Yamaha. It was aimed at the low cost market. It uses frequency modulation synthesis to generate sound, and has 9 channels with 2 operators each. Its follow-up was the popular YM3812 (OPL2), to which it was very similar - the OPL2 added only the possibility of using 4 different waveforms instead of only sine wave, and was otherwise backwards compatible. The YM3526 (OPL) was used in a Commodore 64 expansion, the Sound Expander which was never very popular, as well as several Arcade games, such as Bubble Bobble.
A very closely related chip is the Y8950, or MSX-AUDIO, which was used as an MSX expansion. It is basically a YM3526 (OPL) with the added capability of recording and playing ADPCM samples using up to 256KB sample-RAM and 256KB of sample-ROM. The simpler and incompatible YM2413 was more popular though.
Programming information about the OPL series sound chips. Has very little on the YM3526 though.