Math Olympiad - Problem of the month http://www.moems.org/zinger.htm
Math Olympiad - contest problems http://www.moems.org/sample_files/SampleE.pdf
Grade 5 Challenge Problems http://www.mystfx.ca/special/mathproblems/grade5.html
Thinking blocks http://www.thinkingblocks.com/TryIt.html
Powerlines1 game http://www.oswego.org/ocsd-web/games/Powerlines/powerlines1.html
Powerline 2 game http://www.oswego.org/ocsd-web/games/Powerlines/powerlines2.html
list of great games http://resources.oswego.org/games/
Pattern blocks puzzles
Chaos Theory http://library.thinkquest.org/3120/
Absurd Math- Pre-algebra from another dimension http://www.learningwave.com/abmath/
Problem-solving games list http://www.onlinemathlearning.com/problem-solving-math-games.html
Math Problem of the Week http://looneymathconsulting.com/problemsoftheweek.html