MMC1 - MMC1 boards. NES-SxROM
MMC2 - MMC2 boards. NES-PxROM
MMC3 - MMC3 boards. NES-TxROM
MMC5 - MMC5 boards. NES-ExROM
MMC6 - MMC6 boards. NES-HxROM
Discrete - The Nintendo-made boards using discrete logic.
Custom - The Nintendo-made boards using custom mappers.
Other NES Stuff - NES carts and things that didn't go anywhere else.
Second Party - Various Nintendo Licensee boards.
Bootleg Multicarts - Various NN in 1 carts in the NES form factor.
Bootleg Multicarts - Various NN in 1 carts in the Famicom form factor.
Single Game Pirates - Various single-game carts.
Complete Tengen - All mappers and cart boards (I think).
Complete AVE - All mappers and cart boards (I think).
Codemasters / Camerica - All of the Camerica stuff that I know of.
Complete Colordreams, AGCI, Bunch Games, and Wisdom Tree - One mapper, tons of boards.