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).
MEI
Mr Barton Maths
Secondary teacher Craig Barton has a site full of useful resources.
NRICH
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.
Plus Magazine
A free online magazine, regularly updated with stories of mathematics in the real world. Very accesible.
PM Theta
The website of the wonderful Anne Watson and John Mason.
Puzzle of the Week
The creation of Andrew Sharpe, this site contains a number of free puzzles. Ideal for a maths week display or activity.
Resourcaholic
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.
TES Maths Community
Source of interesting debate and discussion between teachers. Tends to be mostly secondary but some primary issues are discussed too.