HDTV antenna tests
We tried out three indoor antennas in a shotgun-style apartment in the metro New Orleans area, which is fairly flat. Antennaweb.org found 16 broadcasters within 9 miles (14 km), all roughly in the same direction relative to our location. The site told us we could get up to 30 channels (some stations have multiple broadcast channels), though it didn't provide info on which stations offer HD.
With each of the three antennas, our TV set's autoprogram function found 26 channels, but we went through each channel to see which ones really came in and which were too weak to keep a consistent picture. All three antennas picked up HD broadcasts of all of the major networks (ABC, CBS, NBC, Fox and the CW) as well as PBS; we also picked up movie channels like THIS and Bounce, as well as a few channels from the Home Shopping Network. Local broadcasters in New Orleans tended to feature long-format ads for local businesses (often based around restaurants) or religious programming.
Disclaimer: Your mileage will vary greatly based on the topographical features of your area as well as nearby structures. And another note: The manufacturers all claim reception distances of 25 to 50 miles (40 to 80 km), but in our testing, anything beyond about 9 miles (14 km) didn't get picked up because of obstacles in the signal's path. If you live out in the flat countryside with great sightlines to broadcasters, you could potentially get stations as far away as they claim.