That's a very general question to answer in a specific way. If you only want the cheapest device capable of displaying an HD signal, a direct view tv is the way to go, specifically a CRT model around 30" or under. Those will display 1080i but will have to scale a 720p image. For a bit more you can get an LCD with native 720 resolution, and a bit more than that will get you a plasma.
If you want a large picture (again, vaguely defined) a projector gets more attractive. Depending upon the brand and the quality level you'll accept, a set over 50" starts to get expensive. A good plasma over fifty inches can cost several thousand dollars, and the higher quality 65" sets from Sony, Pioneer or Mitsubishi can set you back $2500 or more, too.
If you want a truly large image, say 80" or bigger, a PJ is the only game in town. Digital projectors, such as DLP or LCD, have dropped in price to where you can routinely get superb quality HD performance for well under $2k. In the LCD market both Panasonic & Sanyo make PJs that have been widely praised by the video cogniscenti. For those who prefer DLP, true HD performance is slightly more expensive but still attainable for less than $2,500. True, you need a screen, which can cost between a couple hundred and a couple thousand. And PJs need pretty good light control (ie they need to be in a room that's pretty dark).
If your viewing is mostly "serious" movie watching, in a darkened room, front projection can be magical. If you like to be able to have buddies over to watch sports, a RPTV or direct view set probably makes more sense. I'm an FPTV guy. I can't imagine ever going back to a big ol' hulking box!
