The KWorld PlusTV ATSC 340U is a USB TV tuner. It can receive both over-the-air digital signals and unencrypted QAM signals (which are delivered via cable). Either way, it's your chance to watch and record HDTV on your Windows PC. Newegg is selling the 340U for an impossibly cheap $14.99 (after a $20 mail-in rebate). Shipping will run you six bucks.
Keep in mind that this is strictly a digital tuner, meaning if you plug in a cable-TV cable, you'll receive only QAM stations. (If you're not up to speed on QAM, check the Wikipedia entry.) What's more, Windows Media Center should be able to detect the tuner if you use an antenna (a small one is included), but it definitely won't if you go the QAM route (that's a WMC limitation). Fortunately, KWorld bundles Arcsoft's surprisingly decent TotalMedia software, which provides full DVR capabilities -- and QAM support.
This is far and away the lowest price I've seen on a digital tuner, so if you want TV on your PC, act now: The deal ends June 30.