Freerice is owned by and supports the World Food Programme. For every correct answer 10 grains of rice are donated through the World Food Programme. Users can answer questions in several categories including two on Maths, one on multiplication and one on basic Maths. It is possible to play at various levels, so choosing level 10 for example means that questions on fractions and directed numbers will be included. Students can sign up (under 14s need parents permission) or login easily with their Facebook account to track their totals and join and create groups. Teachers can register a class, so this provides a way for teachers of much younger students to sign up their students.
Another site which includes a social aspect is Sporkle which has an extensive collection of timed quizzes in many subjects, many mathematics quizzes are available; I have linked to some here on Mathematics for Students. If you register you can create your own quizzes and give the link to friends. It is then possible to see how people get on with your quiz!
And last but certainly not least is a site which I came across recently which I think lives up to its name, Brilliant! Signing up to this site allows users to join an international community and get free weekly, personalised problems. Questions at various levels are available, including questions suitable for younger students.
This would be an excellent site for any students preparing for participation in Maths challenges, particularly by using Brilliant’s Techniques Trainer. Brilliant are currently developing a “Teacher Level” that has a wider range of problems that can be shared which would make it possible to share problems from a number of different levels with students. (Post with link to Brilliant for students).