-    Ed Pegg Jr.'s MathPuzzle

-    Ken Duisenberg's Puzzle of the Week

-    Cut The Knot (by Alexander Bogomolny)

-    Dennis E. Shassa's Puzzling Adventures (in sciam.com)

-    Patrick De Geest's World of Numbers

-    Cihan Altay's PQRST Puzzle Competition

-    Yilmaz Ekici's TurkZeka Puzzle Competitions

-    Brain teasers by Barry R. Clarke

-    Lloyd King's Creative Puzzles

-    Clifford Pickover's Puzzles

-    Erich Friedman's Math Magic

-    Eric Weisstein's World of Mathematics

-    On-line Encyclopedia of Integer Sequences

-    Logical and Mathematical Puzzles

-    Nick's Mathematical Puzzles (By Nick Hobson)

-    Puzzles of Leonid Mochalov

-    Puzzle Master - Large and unique collection of brain teasers and puzzles.

-    PhD certified Aptitude and other Self-Assessment Tests