Yes, you can buy a HD tuner and watch DTV signals on your analog TV set - in analog, of course - via s-video. Then, you can use it when you buy a real HD TV, or use it on another analog TV set. Whatever!
Why would you want to do this? Well, for instance, our local PBS provider has 6 channels of programming available on DTV vs. 1 channel on analog.
Besides, one of these days, everyone with an analog TV is going to have to buy one of these DTV converters, so why wait?
I use a Motorola HDT101. It's been great! Highly recommended!