The NRICH website is rightly recognised as one of those "can't do without" resources for Maths teachers.
SYMH encourages all teachers to search for relevant resources to support them in developing both reasoning and problem solving skills.
As well as resources both paper based and interactive there are thought provoking articles that address key aspects of mathematical learning.
For example the article on reasoning can be found via this link http://nrich.maths.org/11336
A new website WILD.MATHS http://wild.maths.org/ has also been launched by the NRICH team, aimed specifically for Maths Clubs and independent research into interesting and stimulating topics. We strongly recommend teachers bring this to their students' attention.
For teachers and older students is the PLUS+ magazine website that is well worth a look. https://plus.maths.org/content/