I use SiliconDust HDHomerun Prime - it is a cable card digital tuner (3 tuners) that works over ethernet to all computers on the network. They have the ATSC/Clear QAM versions as well.
They will be releasing the 4 tuner version shortly.
Ceton also has a good 4-tuner version - both PCI and USB - the difference is they must be dedicated to a PC - if sharing tuners isn't a priority - these tuners are great.
Hauppauge also makes good tuners - both cable card and ATSC/Clear QAM - again, they do not share tuners. They have 2 tuners in their cards/usb devices.
So which one is best?
1) If sharing tuners across a network - go with SiliconDust.
2) If not sharing tuners - do you need 4 tuners? Ceton
3) If not sharing tuners - do you need 1/2 tuners? Hauppauge
The second question - is this for over the air, cable, or digital cable. All 3 companies will support all 3 methods, however, the ATSC/Clear QAM are about half the price of the Digital Cable Card tuners.
Last question - internal vs. external. If you have a good power supply that has enough voltage to support the card, and good ventilation in your computer, the internal cards support a "cleaner install" - in that less wires and devices outside the PC. The cards generate a good amount of heat. The external devices either through ethernet or USB will not tax system resources for power or generate additional heat in the computer case.