Waldomaths by Ron Barrow has extensive resources for students to explore. These work well on the interactive whiteboard.
I like using these in class as students can then experiment themselves at home.
A recent addition is this applet on fractions which allows students to study examples and see how to handle fractions.
The fractions A and B can be changed and students can see how to add, subtract, multiply or divide the two fractions.
I have used and like many of these applets - Circle Theorems for GCSE for example or the Normal Distribution in KS5 to show how the probability changes for different areas under the curve. (I have used this together with a Normal Distribution calculator – see link on this page).