The Mandelbrot filter can be activated from the Render filters category.
The interface lets you zoom into a specific area of interest. You can also set the number of iterations (Depth), both for interactive previewing and a different value for when you let go of the mouse. (they can of course also be set to the same value, so that what you see while browsing through the fractal is the same as the final render).
You can change the gradient that's mapped onto the fractal.
You can also add new layers for additional compositions.
This new filter lets you explore. And once you think you're done exploring, continue discovering new patterns and shapes by combining the result with additional filters and enabling such things as mirror mode.
click to enlarge
Here are a few more examples of where this will take you:
combined with mirror: (watch the video at the bottom of the page)
Fractals from space (used with new Spherize tool)
Here's a video using it along with mirror mode, and then mapped to 3D Perspective (with mirror still enabled) for a SlitScan effect