Natural Maths: creative maths lessons for ages 3-16. Electronic books under Creative Commons.
Maths Circles Ireland downloads: lesson plans with puzzles, games, problems and solutions for ages 8-18.
Khan Academy online maths lessons and practice for ages 5-18.
MathHomeSchool network, lessons and problems for ages 6 - 13 ;
nrich.maths.org puzzles/problems for ages 5-18, online and printable games and activities, posters;
Alcumus online problem solving for ages 8-18, with solutions and accompanying video lessons.
UKMT Junior Mathematical Challenge problems for ages 12-15, with short solutions
Virtual Maths Club problems for ages 11 - 18, with solutions;
Kangaroo problems for ages 6-18;
Paul Erdős International Math Challenge problems for ages 8 - 15;
Purple Comet problems for ages 11-18, with answers;
Cut the Knot puzzles and problems for ages 13-18, with explanations.
The US National Association of Math Circles resources with problems for ages 13-18.
Olympiad related lecture notes for ages 15-18: UCC site and UCD site