http://illuminations.nctm.org/Activity.aspx?id=3565
http://www.fuelthebrain.com/Interactives/app.php?ID=29
http://www.mathplayground.com/puzzle_pics_addition.html
http://www.mathplayground.com/ASB_Canoe_Puppies.html
http://www.sheppardsoftware.com/mathgames/monkeydrive/addition/MDAddDoubles.htm