All the major networks broadcast HD over the air. And Comcast carries them. But they don't tell you this and most of the customer service reps don't know how it works. So how do you take advantage of this?
First off, make sure your HD TV has the option to delete channels. Then plug your cable directly into your TV and hit channel search. It will pull in all the analogue and digital channels. Then go to the delete channel function. Many times you will see a channel listed several times. Like 2, 2.1, 2.2, 2.3. In this case, 2 is analogue and 2.1 is HD for the same channel (2.2 and 2.3 are bonus channels you won't get on your cable box). Delete 2, but leave the others. Walk through all the channels doing this. If you see just a number by itself, like 20, leave it alone. Now everytime you change a channel, you will get the HD version if it is available.
If you also have a cable box with On Demand, here's what you do. Install a high quality cable splitter to the cable coming out of the wall. Attach two cables. Plug one directly into the TV and follow the procedure above. Plug the other one into your cable box. Using RCA cables, or HDMI cable, plug that directly into the cable box and the other end into the appropriate input into your TV. Viola! Now when you want to watch regular broadcast on your TV, just use the normal TV input (in setup it will be Antennae / Cable). When you want to watch channels only available on the cable box, select that input.