A small "getting started" tutorial for OpenCL. See http://www.thebigblob.com/getting-started-with-opencl-and-gpu-computing/ for more info.
For simplicity, the main.c in this directory does not check for errors. If it doesn't work for you, try the version in the 'errorchecking' subdirectory instead, which does (this was contributed by Christian Jaeger.)