The YM2413, aka OPLL, is a cost-reduced FM synthesissound chip manufactured by Yamaha Corporation and based on their YM3812 (OPL2). To make the chip cheaper to manufacture, many of the internal registers were removed. The result of this is that the YM2413 can only play one user-defined instrument at a time; the other 15 instrument settings are hard-coded and cannot be altered by the user. There were other cost-cutting modifications: the number of waveforms was reduced to two, and the channels are not mixed using an adder; instead, the chip's DAC uses time-division multiplexing to play short segments of each channel in sequence, as also done in the YM2612.
Yamaha YMF281 is another YM2413 derivative. It has the same pinout and register set, but a different sound bank.
UMC U3567 is a 100% software compatible chip with a slightly different pinout, requiring a pin-to-pin adapter in order to be used on a YM2413 socket.
UMC UM3567 is a 100% software compatible chip, has the same pinout of the U3567 but a slightly enhanced DAC output. It requires the same pin-to-pin adapter, but the different DAC output will also require two resistors to be removed from the motherboard.