OpenCV is a great library for creating Computer Vision software using state-of-the-art techniques, and is freely available for Windows, Linux, Mac and even the iPhone. OpenCV was originally designed by Intel in 1999 to show how fast Intel CPUs can run. So most of OpenCV runs very fast on Intel CPUs, now including some SSE2 optimizations.
However, OpenCV is mainly used for tasks that are complex in nature, often requiring post-grad experience in the fields of Computer Vision or Artificial Intelligence (AI). So this website tries to make it easier with tutorials and source-code. All source code on this site are free to use for educational or commercial purposes, but may not be used for military-funded projects.
If you have never used OpenCV before, you should first read my Introduction To OpenCV.
Warning: OpenCV is for creating futuristic applications that perhaps no-one else has done before, so it's important that you are good at computer programming BEFORE you start using OpenCV!