IBM Research: Ponder This
- Monthly logic and math puzzles set by IBM researchers with solutions.
- http://researchweb.watson.ibm.com/ponder/

Kakuro-san
- Providing graded Kakuro puzzles every day, plus old puzzles.
- http://www.kakurosan.com

CellFighter
- Shareware game based around an extension of the game of life.
- http://www.cellfighter.com/conways-life-download.html

Leisure Ideas: Puzzles
- Some mathematical puzzles with answers and/or a hint.
- http://www.leisureideas.com/logic%20puzzles.htm

Monty Hall Game
- JavaScript example of the Monty Hall Game. This game demonstrates the real (empirical) affect of statistics. [Does not display correctly in all browsers.]
- http://montyhallgame.shawnolson.net/

Quick Logic Puzzles
- Use the clues provided to deduce the correct sequence of numbers in the hidden code.
- http://www.robertwilliams.org/qlp/index.html

Two Envelope Paradox - Solution
- Claims that the two envelope paradox comes from the incorrect use of probabilities.
- http://www.anc.ed.ac.uk/~amos/doubleswapsoln.html

WU Forums
- Discussion board hosted at Berkeley with talk about a variety of solved and unsolved brain teasers.
- http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_hard

Mathematical Jigsaw Puzzles
- Information on polyominos and square puzzles.
- http://home.hetnet.nl/~paslen/jigsawpuzzles.html

The Monty Hall Problem
- A probability theory puzzle with solutions. [English and German.]
- http://www.remote.org/frederik/projects/ziege/

Perplexus
- A forum and collection of brain teasers, logic problems, math problems, paradoxes, science games, probability, riddles, and geometric shapes. The puzzles are searchable and categorized, most have solutions.
- http://perplexus.info/

TechInterview
- Answers to technical interview questions, programming problems, and brain teasers.
- http://www.techinterview.org/

The Truth Tree's Math and Logic Board
- Collection of math and logic puzzles ranging in levels of difficulty.
- http://www.truthtree.com/mathdoor.html

Jim Loy's Puzzle Page
- Classic and original puzzles.
- http://www.jimloy.com/puzz/puzz.htm