The sky is your limit for inspiration. Check out the applications section of the Wikipedia GPGPU page: http://en.wikipedia.org/wiki/GPGPU#Applications

Molecular dynamics, quantum chemistry?

If you really want to donate, there's always folding@home. Of course, a render farm can do the trick too if want purrty ray tracing? Look into Blender.

Machine learning? Implement pagerank or SVMs on CUDA :)

What is the mapreduce of CUDA going to be?