Introduction
maths.org is connected with the UK's Millennium Mathematics Project (MMP) which is a collaboration between the faculties of mathematics and education at the University of Cambridge. It provides the launching point into several other sub-sites and associated sites which provide different resources for teaching and engaging students in mathematics.
Purpose
This site (and it's associated sub-sites) provide free resources for engaging students in mathematics. They are consistent with the problem-solving approach to learning mathematics and cover many different topics in an engaging manner. I see the most immediate value for the classroom in two sections in particular: https://nrich.maths.org and https://sport.maths.org.
NRich
One feature I really like is the "Live Problems" (example: https://nrich.maths.org/8516) that they provide. These are a collection of problems that are positioned for different academic levels that are designed to inspire creative problem solving. The questions stay live for a period of time (6 weeks) and they encourage students to send in their solutions. They curate the submitted solutions and publish the ones that they think provide good value for learning and understanding.
In addition to the Live Problems which I see as "Big Think" problems which will take the students some time to solve, there are also many smaller problems on the site which are categorized by applicability to age as well as by type of math being addressed (e.g. geometry, statistics, measurement, etc.).
Sports
The https://sport.maths.org/ site has resources similar to those of the nrich site but it is designed to develop problem solving and mathematical reasoning for students at all levels up to grade 12. These problems may be attractive for students for who sports are an interest. The activities are broken up by sport or by age category.
https://sport.maths.org/content/help#activity
Relevance
This content is diverse and engaging and is great for a hook to engage students in the topic of an up-coming lesson. Some of the longer or live problems would be appropriate for challenging students who are ahead of the other students and looking to go deeper into an area or as bonus questions for students to demonstrate the full extent of their understanding.
Positives
There is a lot of material to draw from and it covers all age groups up to the end of high-school.
Negatives/Limitations
Unfortunately, video clips which were provided through collaboration with the BBC can not be viewed in Canada and there is no direct connection to the Ontario curriculum so finding the questions which are relevant to a specific class may be difficult.
Reliability
The site is a project of the University of Cambridge and as such has a high level of credibility.