Stenciled shadow volumes provide a GPU-accelerated basis for shadow determination. Learn from this SIGGRAPH 2004 presentation (newly salvaged from my bit-attic) how OpenGL features such as stencil testing, wrapping stencil operations, two-sided stencil testing, and depth clamping (now a standard part of OpenGL 3.2) provide the API necessary to optimize shadow volume rendering in your 3D application.
http://www.slideshare.net/Mark_Kilgard/realtime-shadowing-techniques-shadow-volumes
I hope this helps.
- Mark Kilgard