The FCC requires cable companies to pass an unencrypted digital feed of your local channels over the cable. you should not have to pay extra for that. However, if your cable company is like mine they will disavow any knowledge of any such thing.
To receive the unencrypted local digital HD feed, you must have a tuner capable of tuning QAM modulated signals. Over-the-air (OTA) HD uses ATSC modulation standards, so a plain OTA tuner will not work. My Panasonic plasma has a built in ATSC/QAM tuner, so I can get the six local channels HD feed over cable and just a basic cable subscription.
To find the QAM channels, I had my TV do an automatic search of all available channels. Worked great! Local channels with HD feeds here are 4, 5, 6, 10, and 12. If I had a cable box, they would be channels 504, 505, 506, 510, and 512. My TV tunes them as 0-504, 0-505, 0-506, 0-510, and 0-512, respectively.
Hope that makes sense.