Rich, challenging problems and some longer investigations. This is a UK-based site. Key Stage 2 is equivalent to Grades 2, 3, 4 and 5.
Challenging math problems worth solving. Organised by grade level and content strand. Tend to be shorter problems.
Puzzles and games organised by grade level, designed to put your brain in a pickle. Some interesting investigations.
Part of the NRich family. Big, open, creative investigations. Also includes short videos of mathematicians talking about their work.