Do any shields (or similar boards) exist to let the Mega 2560 (or Due) output a full 100mA per pin on at least 24 pins, in a digital on/off fashion? I know there is a motor shield, but it looks to have only a handful of outputs. It seems very common that people need to output higher current than the MCU can provide safely (if at all), but there do not appear to be any shields for this. Of course individual transistors are fine for a few outputs, and some chips can do up to 8, but what about the rest of us who want to use all those nice pins on the Mega, Due, or similar but who need more power?
use a driver chip, e.g. ULN2803 which gives you 8 outputs of up to somewhat 500mA from 8 digital pins
use a serially addressible power+logic chip, such as TPIC6A595NE which gives you 8 outputs, each can drive 350mA, and has a built-in short circuit protection. You can cascade these, 3 chips give 24 outputs, and need only three pins to drive all 24 (or more if you use more chips) outputs.
There are 24 output devices, however not in this current range. 24 output devices are usually i2c or spi based led controllers - some of them gives you goodies like rock solid hardware PWM on all 24 ports - but none of them gives you 100mA output.
You'll likely have to use an https://www.arduino.cc/en/Main/ArduinoProtoShield with 3+ of those chips you like that do up to 8.