You can get a TIVO to do it, although I think you may still need a cable card to get it to work (something about channel numbers according to my friend who does this) and you'd pay a subscription to them. Or you can build your own thing using a media center pc or under linux. I personally use MythTV and a couple of HDHomeruns which gives me 4 tuners and also use a few remote frontends. There is very little out of the box which you can get, also beware of putting too much money into clearqam as Comcast are wanting to lock down those locals, so the whole thing could stop working - at which point, I suppose you could stick an antenna on your roof. (If you went the cablecard route, even when they do lock it down, you'd still be OK).
I assume you're paying the Local channels only option on Comcast (which in my area is cheaper than the 1 service only fee).