The list here is far from complete. There are just too many sites to browse. If you come across any useful websites that you think should be included in the list, do let me know.
Faulhaber - DC motors manufacturer
Brushed DC Motor Introduction - How does a DC motor works!Optical encoder & quadrature signals - What is optical encoder and quadrature signals.
H-bridge introduction - DC Motor H bridge driver introductionPWM & DC Motor Speed Control - Explains the basic concept of using PWM signal to control a DC motor speed.
Servo Motor -Brief introduction to RC servo
Solenoid - For those thinking of designing your own solenoid
Good and bad wiring practice - How to reduce noise through good wiring practice.