Brain Den has great riddles, teasers, puzzles and so much more. We really like the layout and how the site has all its options in an easy-to-navigate menu on the left side of its page. With a forum, you can discuss all the great games you're playing and even ask for help.
Visual Phenomena and Optical Illusionshttp://voillusions.blogspot.com/
This blog's categories include 3D illusions, multiple meanings, impossible objects, and more. Whether you're staring down a black hole or trying to decide which way is up, you're sure to enjoy these mind altering posts.