Elgato (meow?) has released a long-awaited update to their EyeTV series. The 250 series allows cable/antenna recording and comes in a small package roughly the size of an iPod. Simply plug the coax into the EyeTV and then plug the EyeTV into the USB port on your Mac. This device isn't a strain on your Mac, it has a built in hardware encoder that will compress video down to MPEG-1 and MPEG-2. It is available for $199 in PAL/SECAM for the Europeans and NTSC for North America.
EyeTV 250 [Elgato]