First off, let's get the facts out of the way. There is no such thing as a high-definition antenna. Over-the-air (OTA) HD signals are broadcast on UHF channels, so theoretically; you could pull in an HD signal with a bent coat-hanger. Having said that, you have to give Richard Schneider credit. Schneider wasn't happy with current antennas, so he created his own.
Schneider's designs proved effective enough to start a website that now sells 1,400 antennas per month. The most interesting design is the unobtrusive Picture Frame Antenna shown above. The PF7 antenna provides a 6.5 DB gain to help pull in your local OTA HD channels. There are plenty of other antennas on Schneider's website; you can pick up the PF7 for under $30.