Search "eyetv" from the Apple store, you'll get a bunch of selections to choose from, and with a lot of user reviews. Do your research though, they all seem somewhat specific on what they are able to do... some analog, some digital, some cable capable (non-encoded), don't think any that do everything. Make sure you know how you will use it.
I just bought an Elgato Hybrid for my MBP, it gets both digital and analog, but is designed for over-the-air signals. I can get cable but only after it's gone through a cable box, so its basically like connecting a vcr between a cable box and a tv. The hybrid is also super portable, and will connect gaming systems. For me, a perfect travelling companion for the MBP and my kids.
I live 30 miles from NYC and the analog reception sucks, but digital, and especially HDTV (free over-the-air signals) is awesome.... I mean wow. I get about 20 digitial channels. The set-up is very antenna sensitive though (direction and amplification).
I personally would not use my MBP for a TV replacement... I see TV as a dispensable entertainment/luxury - the computer is a necessity. I think the added wear and tear of always using the computer as a TV, especially the very disk intensive recording/playback stuff, would not be worth risk of killing the computer.
