An active community has evolved that focuses on GPGPU applications. GPGPU stands for general-purpose computation on a graphics processing unit. The web site http://www.gpgpu.org is a hub for this area, and does a fantastic job of cataloging the current and historical use of GPUs for general-purpose computation. Lately, a few seminars have covered this topic (GP2 and SIGGRAPH 2004, for example), and a full-day course at SIGGRAPH 2005 will focus on GPGPU.
If you're interested in diving right into GPGPU, read the recent book GPU Gems 2: Programming Techniques for High-Performance Graphics and General-Purpose Computation (Addison-Wesley, 2005, ISBN 0321335597), by Matt Pharr and Randima Fernando. The book focuses on the GPGPU space, devoting 14 chapters to this topic, covering everything from fundamentals to optimizations to real-world case studies. Starting by tackling the trends and underlying forces that drive streaming processing forward, plus detailing the low-level design of a modern GPU (excerpted here), the authors give readers a strong base in GPU fundamentals. Building upon that base, readers learn more thoroughly the detailed differences between the CPU and GPU, strengths and weaknesses of each, and how to most efficiently map computational concepts to the various portions of the GPU. Efficiently harnessing the GPU for computation is then discussed, with focus on parallel data structures, flow-control idioms, and program optimization.