On the left is Time-Warner's cable modem
and on the right is the one I  purchased
for $20 on eBay. This was shot before
the changeover and the "old" modem
is still providing the Internet service.

When Time-Warner announced in the fall it would start charging $4 per month to lease me the modem that was already in my house, I decided to buy my own cable modem and stop paying this extra charge.
It turns out there are quite a few units that will work on TWCNY's Roadrunner system.
The modem that's installed in my house, a Motorola Surfboard SB5101N is a common model that's served me well, so that's what I shopped for.
Amazon has new ones for about $47; it would pay for itself in a year.
But what about a used one? I turned to eBay and snapped up  an SB5101, which my research has shown is functionally the same, for $20 (I've since seen them for $14).
At $4 per month, it won't take long to pay for itself.
Hooking it up was easy.
I plugged it in to an outlet and moved the cable and ethernet connections from the old box to the "new" box (be sure to screw in the cable connector snugly).
At this point you'll lose Internet service.
I made note of the modem's MAC address and called Time-Warner's helpline (I got through quickly for a change). I
gave the technician the MAC address, she punched some computer keys on her end, and in a few minutes my Internet was back on using my modem.
It seems as speedy as ever.
Now all I have to do is drive to Rome and drop off Time-Warner's modem and my cable bill will go down $4 per month.

I've circled the MAC address found
on the bottom of TWCNY's  modem.
The other MAC ID is for a USB port this
 particular model happens to have
(my "new" modem doesn't have one,
but I have no  need for this feature).

Below are some links I used.
Time-Warner's instructions on how to do this, including a lit of modems that have been tested:
http://bit.ly/TcYzLM

The Surfboard SB5101 on Amazon:
http://amzn.to/XNHzyB

The Surfboard SB5101 on eBay
http://bit.ly/140cHyD

Please note the modem I bought was a DOCSIS 2.0 model, suitable for most typical home accounts. If you're shelling out the extra bucks for one of the high-end data Internet plans, you'll need a DOCSIS 3.0 modem, which will cost you at least twice as much.
Since Time-Warner charges the same amount to lease one of these, and will replace it for free if it goes bad, leasing might not be such a bad deal if you need one of these.