I would like to point out a fantastic CUDA accelerated app called Fractron 9000. Google for it. It’s freeware, but not open source - and it’s Windows only (based on .NET Framework I believe). The author doesn’t seem to advertise this program anywhere - so I’ll jump in and do it ;)
This tool renders so called fractal flames which are based on iterated function systems (IFS). It renders these fractals by means of playing the chaos game. I.e. it iterates starting with randomly chosen points and counts how often each screen pixel was visited. Then it converts the result into an image using a tone mapping algorithm. The longer you render, the less noisy the factal will become - but you can follow the rendering process in real time which distinguishes this software from many others I’ve seen
This software “only” offers 5 variations of linear and non-linear transformations that can be arbitrarily combined (other tools like Apophysis offer way more, but then they aren’t nearly as fast). This is the fastest flame renderer I’ve seen, hands down.
One particularly nice feature of this program is the editor which allows you to edit the linear transformation that is applied to each branch of the fractal with immediate visual feedback. The default view consists of three strictly linear branches, arranged to form the famous Sirpinski Triangle. To get started, drag around the transformations (indicated by circles with handles that you can click your mouse on) and inspect the visual effect. Also try rotating these for best effect. Then proceed to load some of the sample fractals that ship with the program.
Coolest use of CUDA I’ve seen so far. I could play with this for hours.