Apophysis is a "fractal flame generator", and quite simply one of the most fascinating applications I've played with in quite some time.
I'm probably a bit late to the scene with fractals, and doing a google search for "apophysis fractals" will yield a literally dizzying array of different images, some absolutely fantastic and some, well, not quite so much.
Still, Apophysis is one of those programs you can sit down with and literally not look up for, say, 3 hours.
I've just started to play with some of the concepts and I'm nowhere near the whole post production effects that many are getting into, but I've managed to create a few unique and interesting images, none-the-less.
For instance, here's a thumbnail of one I call Solar Flare:
Check out the rest of them here.
And hopefully, I'll be posting more as time goes on.