You should look at elgato products for macs, I've had a usb dongle for Freeview and the EyeTV software is the best around, you'll love it.
Some external tv options can accept video in, which is displayed through EyeTV, but these are small boxes, not dongle form. Again these are from Elgato
*Look on Apple's site under Video Devices, some of the bigger video devies which accept video inputs are there. Elgato 400 I think is the one?