The Integrating Vision Toolkit (IVT) is a powerful and fast C++ computer vision library with an easy-to-use object-oriented architecture. It offers its own multi-platform GUI toolkit. OpenCV is integrated optionally. Website: http://ivt.sourceforge.net
Veejay is a visual instrument and real-time video sampler. It allows you to "play" the video like you would play a piano. While playing, you can record the resulting video directly to disk (video sampling).
GSVideo is a cross-platform library for the Processing programming language that provides video support (movie playback, video capture, creation of movie files) through the use of the GStreamer multimedia framework.