Sometimes, HBO, Showtime and similar have freeviews. They are not the channels themselves anymore, but selected shows and movies OnDemand. Comcast rarely, if ever, announces it. The last one was posted here on the last day of the freeview!
Comcast also used to have an annual week-long Watchathon, which involved many networks, but I have a feeling it's dead now that most networks have alternative ways of getting people to watch outside of cable.