I started writing mightymandel, a Mandelbrot Set explorer using OpenGL 4, back in January, but only got around to beating it into proper shape this week. I'll write more about how it works another time, suffice to say that my GPU is much faster than my CPU.
You can get the GPL3+ code here:
git clone http://code.mathr.co.uk/mightymandel.git
Note that you need the literate preprocessor anansi to unpack the single-file source (which is sadly undocumented so far).