mandelbrot explorer
Its a toy, but it uses OpenCL to render with. Its fast has a bunch of options for zooming / quality / renderer type.
The default renderer is OpenCL but it can use the double or bignum rendereres both are spread across multiple CPU threads.
You can control the quality and depth of the image by changing the max / min iterations.
Zooming and point and click work fine, zooming out just takes you to the previous level with no rendering required.