January is National Puzzle Month, and Jan. 29 is National Puzzle Day.  And since puzzles challenge students and help create new thinking skills, puzzles and riddles of all types can do wonders for students working individually or in groups.  Try these excellent sources for free materials.

Puzzles

Activities for ESL Students
Includes crosswords and many other resources, too

Celebrate National Puzzle Day

Chess Puzzles

Math.com Puzzles, Games and Contests
Some are interactive.

A Puzzle a Day Provides Practice that Pays

The Science Spot Puzzle Corner

Word Find Puzzles for ESL Students

You Don’t Need to Change How You Teach to Improve Assessment Scores
Puzzle and quiz making sites

Riddles

About.com Riddles
Large link page with more difficult riddles

AzKidsNet Riddles
Riddles for kids -- silly, but they'll share them for years.

Brain Food:  Riddles
Riddles for older students

Brain Teasers, Puzzles and Riddles

Classic Riddles

Einstein’s Riddle
Not for the meek

Famous riddles

Just Riddles and More
Riddles by subject 

The Riddle of the Sphinx
Perhaps the most famous of all

Riddles
More terrible riddles

Riddles.com
Major riddle site – interactive with riddles, brain teasers and puzzles