I made this way back in middle school in Adobe Flash CS4. (This was before Adobe changed the name.) It is a Windows executable that animates sectors of a circle being rearranged into a pseudo-parallelogram. This helps you visualize why the formula for the area of a circle works.
I also had a website in middle school. It was a different website than this one. The text below was taken from an archive of my old website.
The Circle Area Visualizer dynamically visualizes the area formula of a circle. The user drags the center and a point to form a circle, which is automatically sliced into the given number of sectors. By dragging the sliders at the bottom or using the animation buttons, the user can rearrange the sectors into a pseudo-parallelogram, from which the area formula of a circle is derived. Below is the algebra behind the area formula of a circle, based on the parallelogram attained with the method that Circle Area Visualizer illustrates.
The current version of the Circle Area Visualizer is 1.2. Download the Windows executable!
Note: The screenshots below were taken with Version 1.1 of the program.