If you bought your TV within the past few years, and you live anywhere close to a city, chances are you can get better HDTV than you've ever seen, for free.
The key, of course, is you need one little thing that you probably already have.
All TVs sold in the United States after 2007 are required to have an ATSC tuner. TVs larger than 36 inches have had ATSC tuners since 2005. So if you bought your TV after this, you probably have a tuner.
The next step is getting a signal to it. Most of you reading this will remember the good old days of screwing on the thick coaxial cable to the back of the TV. Same thing here. If you still have an antenna in the attic, give it a try. It will probably work.
Digital signals are rather finicky. I remember watching channel 56 in Boston growing up, and it would barely come in, with lots of snow and noise. How that infuriated my parents ("you'll ruin your eyes!"). Digital signals aren't like that. Check out my article on how HDMI cables work (and how all perform the same), because over-the-air digital broadcasts have a similar perfect-or-unwatchable failure. You either have all the signal, and it's perfect, or you fall off the digital precipice, and you've got dropouts, visual artifacts, or just a blank screen.
The problem with digital OTA signals is there are a number of factors that could lead to poor signal quality in your home. If you're in a big city, the buildings can cause a problem. If you're way out in the country, distance could be the factor. Just because you had watchable channels in the old days, doesn't mean you'll have watchable digital channels.
The tuner in your TV is also a big factor; some are better than others.
In most cases, you can get better signal quality by either turning your antenna, or getting a better antenna.
The CEA and NAB have an excellent Web site (antennaweb.org) to help ensure your antenna is facing the right way, and to give you pointers on what kind of antenna you need. Also check out the FCC's page on Antennas.
You'll also get a listing of all the channels in your area. Once you figure out what antenna you need, Amazon sells them, as do places like antennasdirect.com.
If you're really hard-core, check out TVFool.com. This awesome site has beautiful maps (image at top and below) and takes into account mountains/line-of-sight from the transmission towers. If the basic maps of Antennaweb.org aren't detailed enough for your area, TVFool should help out a lot. It's also nerdy-cool, and I know nerdy-cool. Here's a color-coded map of KNBC's coverage in Los Angeles. Sorry Burbank, no OTA Jay Leno for you.
Unfortunately, even if you can get digital channels, it's not a guarantee that they're HD. That's up to the individual broadcast stations. All have to be digital, but they don't have to be HD.
Most stations will broadcast one HD signal, and one or more SD stations within their allotted bandwidth. So it's possible to have many more channels available than the old broadcast days, even if the total number of broadcasters hasn't increased.
Many TVs have different picture settings for different inputs. Unless you can convince your local broadcaster to transmit test signals (not totally beyond the realm of possibility), your best bet is to either try to set it by eye or copy the settings from a calibrated input, or both.
But, but, but, my DVR!
If you've become accustomed to the convenience of your DVR (and who hasn't?), the switch to over-the-air may seem jarring. After all, there's no DVR, right? Turns out, there are lots of options.
If you're computer-savvy, you can build an HTPC that will act just like a DVR. I love my PC, but incorporating one into your living room isn't like plugging in a Blu-ray player. Many swear by it though, and I'd love to hear from them in the comments on their best strategy.
For the rest of us, there's TiVo. The latest DVRs from TiVo offer streaming like Netflix and Amazon Video on Demand and Hulu Plus, and are available for less than $100. So free HDTV over the air, plus the pay services for the shows you missed on cable. Not a bad way not to pay for cable. Though who knows if you'll really save money.
Over-the-air reception devices rule
One of the most common complaints I hear from people about getting OTA HD is that their homeowner's association won't allow rooftop antennas. Legally, they're not allowed to have this restriction.
Whether you want that fight with your neighbors is the bigger question.
Check out the FCC's page on the topic here. It goes for apartment dwellers too.