Math Is Fun http://www.mathsisfun.com/ is a free online collection of Mathematics related activities, tools and resources for all school stages. It combines online tools for each area of maths, along with interactive activities, puzzles and worksheets, as well as a maths dictionary, puzzles and games.
Online tools (such as for graphs) and animations showing use of maths tools (such as a ruler and protractor) are useful both in the classroom for demonstrating skills as well as for revision by pupils on their own.
The different ways of accessing the resources means that the teacher can find resources for a specific age group, or for a specific maths skill, or by definition via the maths dictionary. There is an A-Z listing, and a listing by categories, and cross-linking from curriculum links.
In addition there is a Maths Forum for teachers to share or ask for suggestions about teaching maths. And there is a guide for direct linking from specific resources to a class or school online space. Mathematics Teachers, Mathematicians and many others work on the site, and the site states that typically each page is reviewed by at least three qualifiied people (with the millions of users of the site able to send comments or suggestions to improve it at any time.
There are fun maths-related puzzles and games online, as well as changing featured resources and facts. A good site for having in the gallery of tools for teaching maths at any school level.