A few nice Arduino images I found:
Image by smellsofbikes
Arduino-based tachometer. The gray cable with the black widget is an LED/phototransistor pair. The aluminum hat is the encoder, with a hole on one side, counterdrilling on the other to balance it (it may see 10,000rpm) and a setscrew.
Arduino – Hello World!
Image by keith_bawden
My new hobby