Analog cable: almost any tuner card would work. Digital cable (known as QAM): a couple of choices for un-encrypted signals but I suspect you are thinking about a cablecard tuner, one that will pick up all the digital channels that you suscribe to -- not yet. When they show up it will require a new OS and almost all new hardware.
Colin and Allen what are you wanting to do? Are you setting up and HTPC? You can use a card like the Hauppauge PVR 150 for standard cable. But like Steve said their aren't any cards now that will do QAM now.
So if you are wanting to do basic cable with a PCI card they work great. You can even get a two tuner version in the Hauppauge line but if you want to do digital cable then you are out of luck.
Yes I was looking for a cable card to replace the the cable company's decoder. They are available for TVs that can accept a card. It seems that if they can place in in RPT why not in a PC. Maybe some enterprising youg electronics firm can figure out how to do it. I was told that the cable company's cable card is similar to pcmcia. Who knows maybe it is out there now.
Cablecard for the PC is not here yet. Microsoft is working on it for their next O.S. (VISTA) The deal they made with CableLabs requires a secure motherboard, Video card, and probably monitor, possibly cables; none of which exist yet. It looks like the "system" must be certified which means Dell, Compaq, etc .... no homebuilt forseen, maybe no whitebox builders either.