HDTV antennae for over-the-air signals don't exactly cost a lot of money, but if you're the DIY type and would rather make your own than buy, all it takes is some spare cardboard, aluminium foil, glue, and a little time to put it all together.
Over at Squidoo, you'll find a step-by-step how-to to building your own antenna, complete with the pattern for the cardboard you should cut out, instructions for taping on the aluminium foil, and even a walkthrough to connecting the wires in the right places and testing it with your HDTV.
Best of all, the entire design doesn't call for much more than simple household components that most people already have. If the box-style antenna isn't powerful enough, there's even a bow-tie design linked in the article that's also easy to make.
A Free Antenna to Watch Free HDTV [Squidoo]