ABCya - This site was created by a teacher. The games are plentiful and high-quality.
BrainBashers - This site has some silly things (a spirograph, online rock/paper/scissors, test to see how accurately you judge the passing of 10 seconds) and it has activities that truly exercise your brain (an IQ test, Sudoku, a face memory game).
BrainFood - “Puzzles for the brain to gnaw on” Word puzzles, logic puzzles, even joke puzzles!
Braingles - Brain teasers, optical illusions, puzzle hunts, codes and ciphers, and mechanical puzzles.
It’s My Life - A website from PBS that discusses subjects kids deal with every day. Six topic “channels”: Friends, Family, School, Body, Emotions, and Money.
The Challenge of the 7 Cups - Solve riddles to find the legendary lost cup. This is not for the casual riddle enthusiasts. You will be challenged!
National Geographic Games - Build your own universe, explore Hitler’s hidden underground city, break secret codes. There are lots of challenging activities on this site.
Neo Kids - Games, puzzles, quizzes, and interactive diagrams. The interactive diagrams are great study aids. For example, if you have to learn the structure of an atom, you can test your knowledge by dragging labels to an atom.