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). With mplayer displaying the feed, it takes 20% of user space CPU on an AMD64 2GHz.
The image is not very stable (it moves up and down 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. Supported video standards are NTSC (720x480) and PAL (720x576).
Warnings from the kernel "xc2028 2-0061: Error: firmware xc3028-v24.fw not found" must not be fixed as explained on the XC2028 page, because it crashes the driver, or at least it makes the device not work, I don't know why yet.