Archimedes' Laboratory...A high-content, award-winning puzzle site that is divided into six main categories: puzzles & tests, optical illusions, custom puzzles, teachers' resource, curiosities, art & language.

The Art of Crime Detection is an interactive Web activity consisting of an introductory animation, two crime-solving scenarios, and an open-ended drawing tool... provides the student with an understanding of the differences between right brain and left brain processes, an understanding of visual images and vocabulary and their impact in providing descriptions,...

CARET Science Puzzles  are great activities designed by the Centre For Applied Research in Educational Technologies.. each puzzle has two options — one for younger and the other for older students.

Herd a Word, an online word-finding game. It’s pretty simple - piece together words from the alphabetic jumble in front of you.

Intelligence - Mensa, IQ, and Intelligence - favourite links is ranked in the top ten most frequently visited ’Mensa’ websites! Chimera Puzzles; Chimera Homepage; Think & Derive (Logic Puzzles)
KidsHub.org - free online interactive learning games, puzzles, and quizzes Best Regards, Dyann K. Schmidel dyann@schmidel.com

Launchball...from the British Science Musuem.  Students can create a sort of video game (and learn scientific concepts in the process), title it, and post the url.

Line Rider 2.  is the sequel to the popular Line Rider game. ... can be used very effectively to teach physics and other topics.  Students can also have a lot of fun creating and saving their creations.
Puzzle Choice - A great choice of puzzles and games for all ages. Crosswords wordsearches, quizzes, wordplay, brainteasers, anagrams, mazes, cryptograms, number puzzles, online games and teacher resources. Fun and educational.
puzzles.com - This site is about Puzzles, Illusions, Tricks, Toys, and everything around all these Great, Funny, Entertaining, Intellectual, and Educational Things

Ology from the American Museum of Natural History has numerous excellent activities on topics like biodiversity, archeology, and astronomy.

Qtoro is a fun game covering countless subjects.  Students can also create their own games and compete against each other. 

Scratch is an application developed by MIT to allow children (of all ages) to create their own interactive games, stories, and other media with the ease of putting blocks together...designed to help young people (ages 8 and up) develop 21st century learning skills. As they create Scratch projects, young people learn important mathematical and computational ideas, while also gaining a deeper understanding of the process of design.

The Senses Challenge is a series of timed puzzles from the BBC

Traveler IQ Challenge, a game that rewards your knowledge of great cities and tourist traps.

Trebuchet Challenge, an online game that lets you harness your knowledge of physical laws to fling boulders great distances using a .medieval siege engine used to smash castle walls. And now you can let your students play with one in the comforts of your own classroom.

Word Central Here, youngsters can get a leg up on building their own vocabulary while having fun.