That's going to depend on your budget, system configuration, and your video source. For instance, if you're a cable/satellite TV subscriber and have more than just basic cable (that requires a set top box), then I'd recommend one of the new cablecard solutions.
Ceton Corp InfiniTV 4 - Internal PCI-Ex1 card/Four tuners. $299
Ceton Corp InfiniTV 4 - USB - External box/Four tuners. $299
SiliconDust HDHomeRun Prime - External box/Three tuners. $250
Hauppauge WinTV-DCR-2650 - External box/Two tuners $140
If you just have basic cable (no set top box required) or over the air transmissions, then (as rwpritchett states while I type this up), just a simple dual TV Tuner card like the AVerMedia A188 or any of the Hauppauge dual tuner devices.
-Wolf sends