TM5600 USB video grabber
This page explains how the Trident TM5600 chipset works in Linux, in particular with the following product that embeds it: KÖNIG CMP-USBVG6 (bought here 30€).
First of all, Trident seems to be well known for refusing to help coding free drivers and even disclosing information about the chipsets, so that's a good reason not to buy it in the first place.
The driver supporting this chipset is tm6000. The kernel doesn't know the device in particular, but recognizes the chipset: "tm6000: Found Generic tm5600 board".
Grabbing video works fine. You plug the USB, you run "mplayer tv://" and you have the image, it's as simple as that if your kernel was compiled with CONFIG_VIDEO_TM6000 (distribution's kernels are).
The image is not very stable (it moves a bit), but I don't know how stable is the source being tested. There is a green line at the bottom of the image.