The Yamaha YMF262, also known as the OPL3 (OPL is an acronym for FM Operator Type-L), is an FM synthesis sound chip. One is necessary for the MBHP OPL3 Module used for a MIDIBox FM Synthesiser. There is a list of soundcards with YMF262 and/or YAC512_chips.
YMF262 on wikipedia.