The Mandelbrot set.

A Julia set. You can switch back and forth between parameter space and dynamical space.

A section of Newton's method. You can zoom into images and go back to where you were before.

Code editing: you can edit your functions and see the results right away. No need to install a compiler. Just hit run.

Sandbox: you can create arbitrary bitmaps using the "sandbox" functionality. Using the "annotations" function, you can add arbitrary vector graphics, as well.