The VeeJay applications implement a pioneeristical approach to video manipulation in realtime,taking advantage of the high computational power offered by personal computers nowadays. If you're active in the field of media and visual art, dance or scenografy, this software can be interesting and sometimes very useful to your research.
FreeJ is a software that I started writing myself in 2001, collecting video compositing tecniques i developed in theatre and interactive installations. It is still in development, but it's allready usable and offers multiple approaches to realtime video manipulation: can be controlled lively and from a remote console (VeeJay over Ethernet!) or scripted in an object oriented syntax. More documentation about its usage, complete with manuals and a friendly tutorial, can be found online at its homepage on http://freej.dyne.org.
Other tools included in dyne:bolic are useful to be employed in different ways on realtime video, the most remarkable are the PD extensions for video called PDP / PiDiP and GEM which can be scripted in a visual way to build the most visionary 2d and 3d interactive video tools, which can also interact with audio.
The so called PD, which stands for Pure Data, is as powerful as complicated to learn. There is a wide community of wild geniuses creating all kind of visionary audio/visual patches and interactive installations. You can get started from the author pages: Miller Puckette wrote extensive documentation about it on http://www.crca.ucsd.edu/~msp/Pd_documentation/index.htm, and if you don't want to feel alone with it you can get in contact with the mailinglist on http://iem.kug.ac.at/mailinglists/pd-list/.