A favourite site – Mathisfun! is a treasure trove of resources.
There are clear indices; the Index from the Home page takes you to the very useful Index by Year and Subject. Explanations and examples are very clear and each section has additional questions to try with solutions. Look for example at the Algebra Index and choose any topic to see the very clearly presented information and activities.
You can also use the following pages to search :
I often refer to this site and know that students like the clear presentation. I have used everything from Balance when Adding and Subtracting with younger students to working out the inverse of a 3×3 Matrix with the Sixth Form and lots in between! The Dictionary too is very helpful.
Thinking about my current teaching, with Year 12 we are looking at the Binomial Expansion, we can see some clear illustrations here including a rather nice illustration for (a+b)3.
With Year 10, having met Surds for the first time this year, some additional examples will be helpful; as mentioned above – each section has a quiz…
My Further Mathematicians are studying Complex Numbers, some rather nice illustrations can be found here for them.