Here are some fun applications which take advantage of parallel processing using NVIDIA's CUDA architecture:
"GPUGRID.net is a novel distributed supercomputing infrastructure made of many NVIDIA graphics cards joined together to deliver high-performance all-atom biomolecular simulations."
GPUGrid optimizations thread
http://setiathome.berkeley.edu/cuda.php Seti] The classic distributed computing app, for CUDA.
Collatz Conjecture Distributed mathematics
Security / Hacking:
"WPA/WPA2-PSK and a world of affordable many-core platforms"
"RainbowCrack is a general propose implementation of Philippe Oechslin's faster time-memory trade-off technique. It cracks hashes with rainbow tables."
Amazon GPU Instances
Using Amazon's EC2 GPU instances to crack SHA1 hashes is described here
An interesting paper titled Accelerating Reed-Solomon Coding in RAID Systems with GPUs” by Matthew Curry, Lee Ward, Tony Skjellum and Ron Brightwell, IPDPS 2008 is described in this post