stevewyborney.com/2017/02/splat/
buildmathminds.com/
www.mashupmath.com/freemathpuzzles
www.learningtrajectories.org/learning_trajectories (lots of free lessons, videos, resources; sponsored by the Bill & Melinda Gates foundation)
www.peterliljedahl.com/teachers/numeracy-tasks
www.youcubed.org/tasks/
nrich.maths.org/teachers/primary
www.openmiddle.com/