Here's a fun trick Comcast is now playing with in Chicago. For unlimited data, it costs $50 a month if you're using your own modem, but using Comcast's modem, I get a "discounted" rate of $20 a month including the modem rental.

It's their way of charging me to use my own modem (which I believe is illegal), hidden behind the ruse of it being a discount to rent theirs.