I do not have a list currently, but I will put one together. Some of the sensors require specific timing and such, so I've already written specific modules for to make it much easier for the end user (eg: RPi::DHT11, RPi::HCSR04, RPi::WiringPi's servo() etc). Many other sensors require nothing more than toggling a GPIO to read/write, such as hall effect sensors, PIR motion sensors etc.
As far as the Arduino, it kind of is needed. It allows me to test all of the functionality of RPi::I2C... read/write, read/write to specific addresses, and most importantly, read/write block data. Because I can code the Arduino in any way I like, I can add/change the way the pseudo-hardware operates, and therefore enhance the tests in the future.