The Amazing Art Site - - Huge collection of optical illusions, hidden imagery, ambiguous art and upside downs.
Archimedes' Laboratory - - Recreational mathematics including puzzles to make and solve, tessellations, geometric puzzles, optical illusions, and curiosities.
AVAV - - Collection of optical illusions, brain teasers and riddles.
Brain Juice - - An assortment of entertaining logic puzzles with solutions and indicated levels of difficulties.
Brain Teaser World - - Offers riddles, puzzles, optical illusions, and magic tricks.
Brain Teasers - - Brain teasers, riddles, logic puzzles and problems, paradoxes, optical illusions, IQ tests and logical games.
BrainBall - - A daily brain puzzle with 10 different tasks and a variety of statistics on your performance.
BrainBashers - - Brain teasers, puzzles, riddles, games and optical illusions.
BrainBoggled.com - - A collection of math, logic, and word puzzles.
Braingle - Brain Teasers & Riddles - - Includes a huge collection of user submitted and rated brain teasers and riddles. It also has a lively message board and some unique multiplayer games.
BrainVista - - Collection of solved and unsolved brain teasers.
Confusion - - It's a challenge to reach the end of this puzzle maze.
A Game A Day - - A puzzle site featuring a new puzzle format each weekday for each month. Puzzles are fun, educational, entertaining and are also printable.
The Grey Labyrinth - - A beautifully illustrated collection of puzzles, riddles, and paradoxes. Features a changing "unsolved puzzle" plus three years worth of previous puzzles.
Had To Play On - - A community based site with competitions, challenges, awards, a help centre, forums and games.
iRIDDLER - - Challenge your mind with free online riddles/puzzles, logic games, brain teasers and view optical illusions.
Puzzles Without Rules - - Free, updated, original abstract puzzles for advanced puzzle-solvers. Includes current unsolved puzzle and archive with hints and solutions.
Pzzls - - A collection of logic puzzles, riddles and brain teasers.
Rec.puzzles Archive - - An archive of many popular puzzles from the rec.puzzles newsgroup.
Roh'El's Riddles - - Offers brainteasers rated by difficulty and category.
Say What You See! - - Collection of visual brain teasers which can be solved by saying what you see.