I will be adding to this page; in the meantime if you are looking for resources, this index of posts includes sites with many resources.
The Explore series of pages on the companion student blog includes resources which students can use to explore topics themselves.
The I’m looking for… page is popular and lists many websites with resources for teachers.
See also the TES Resources post and note the Secondary Maths page for resources for students age 11-18 and the Primary pages for younger children: Key Stage 1 and Key Stage 2. See the very useful collections page on the Secondary page which organises resources into groups. There are several new Topic Specials where members of theTES Maths panel, of which I am very pleased to be a member have picked their favourite TES resources for teaching a variety of topics.
The Guardian Teacher Network also includes resources for teaching Maths.