For testing, some of the available flash or RAM cartridges are good bets. For distributing in small hand-built volumes, an NES cartridge is really just some parallel storage for the 6502 CPU(be it an EPROM, or a FLASH 29-series ROM (easier)) and another chip for the CHR graphics on the PPU bus (a similar ROM). RAM may be used in place of CHR ROM, and bankswitching "mappers" are commonplace, as the 6502 can not address much as-is.