Here’s a new site (to me, anyway) that i came across somewhat by accident. It is called Nrich Maths (http://nrich.maths.org/public/) and it is best described on the website itself:
The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice. More information on many of our other activities can be found here.
On our website you will find thousands of our free mathematics enrichment materials (problems, articles and games) for teachers and learners from ages 5 to 19 years. All the resources are designed to develop subject knowledge, problem-solving and mathematical thinking skills. The website is updated with new material on the first day of every month. For guidance on how to find the right resources for you, go to the Help section of the site.