At first glance, Fractal Lab isn't immediately exciting -- in fact, when you close the Help dialog, all you get is a blank checkerboard. It's only when you start hitting some buttons that you realize just how awesomely powerful this demonstration is. If you haven't opened the site up yet, do so now. Then hit '3D,' and wait while your CPU renders your fractal design. What you will eventually see before you is drawn using WebGL and the GLSL shading language.
If that wasn't cool enough, you can move around the rendered object! Scroll in and out with the mouse wheel, move around with W and S, A and D. If you have a powerful computer, un-tick 'Preview mode' at the bottom; it'll make everything look rather beautiful.
Then, when you're bored of that, start moving through the tabs: Constants, Fractal, Camera, Colour and Shading. Each and every one will radically change the rendered object, or how it's displayed in your browser. Pick some new numbers (change the 'Fractal type' on the Constants tab!), then hit Recompile.
And yes, if you weren't excited enough about seeing an actual, real-world application of WebGL, you can even save your fractals using HTML5's localStorage feature!
Fractal Lab is a small piece of genius, and hopefully a good sign that more awesome things will soon gush from the HTML5 and Open Web pipeline.