More detail on OpenCV and its scope (Score 1)

OpenCV is Intel's Open Source Computer Vison Library. It's a decent-sized library that helps out with image processing tasks. It's for image processing though - you still have to work out how to capture the video yourself. For that, use V4L (see other comments). One of the included example apps ('CAMSHIFT') includes code to do video capture, but only works under DirectShow on Windows.

If you want to do stuff like extract image features or track moving objects in the video stream, it'll be perfect.

The code is available from the 'opencvlibrary' SourceForge project under a BSD-style license.