OpenCV is a cross-platform, open-source, commonly used computer vision library. It uses C++ as its primary interface, but other languages such as C and Python can also be used. OpenCV was originally developed by Intel back in 1999 and is now maintained by Willow Garage.

OpenCV was originally written in C for maximum performance and portability.
OpenCV v2.0 was released in October of 2009 and includes major improvements in C++ interface.

The OpenCV Installation Guide [opencv.willowgarage.com] is a great way to start.
Installers for Linux, Mac and Windows are readily available there

For more in-depth info about OpenCV there’s a 500+ page book published by O’REILLY about OpenCV 2.0 Learning OpenCV: Computer Vision with the OpenCV Library [amazon.com]

OpenCV main strengths are computational efficiency with a focus on near-real-time applications.
This makes OpenCV perfectly suitable for writing applications on mobile platforms such as robots and cell phones.

Face recognition, Object Identification and Augmented Reality are some of the examples of OpenCV usage.

Related Wiki pages:

OpenCV on Maemo [developer.qt.nokia.com]