1) http://www.hkedcity.net/ (香港教育城 Hong Kong education City)

 
2) http://en.wikipedia.org/wiki/Theory_of_cognitive_development#Preoperational_stage (Jean Piaget's theory of cognitive development)


3) http://www.eyecanlearn.com (visual perceptual skill training)


4) http://www.highlightskid.com (interesting games, e.g. hidden pictures, sciences, art & craft)