Unicap
Website link. unicap provides a uniform interface to video capture devices. It allows applications to use any supported video capture device via a single API.
It supports V4L devices and firewire video devices. It provides an abstraction layer to video streams and devices. The project also provides a library, libucil, providing easy to use functions to render text and graphic overlays onto video images. Finally, there is a GTK widget set for unicap, unicapGTK, making GUI easy to develop.
Unicap is used by GoQat and supports dynamic rewiring using VideoJack (unicap plugin link).