MEI is a charity committed to improving mathematics education and promotes teaching and learning through different strands of activity. They have both free and paid resources (the latter via a subscription service).
Secondary teacher Craig Barton has a site full of useful resources.
This website is from a Cambridge-based organisation, specialising in rich tasks for both primary and secondary school children. An excellent source of problem-solving tasks with notes for teachers.
A free online magazine, regularly updated with stories of mathematics in the real world. Very accesible.
The website of the wonderful Anne Watson and John Mason.
The creation of Andrew Sharpe, this site contains a number of free puzzles. Ideal for a maths week display or activity.
Secondary maths teacher Jo Morgan has spent years curating the ‘best of the web’ in terms of secondary school maths teaching resources. Check out her ever-growing and entirely free collection here.
Source of interesting debate and discussion between teachers. Tends to be mostly secondary but some primary issues are discussed too.