Why should you pay your ISP $8-10 / month for renting a cable modem? Buy a $80-90 cable modem itself. It will pay for itself within a year. Just remember to check you cable bill if the unscrupulous ISP isn't still charging you for cable modem. Then there are ISPs like Charter who bundle the rent in the fees so buying your own cable modem won't save you money.

You don't need to buy a cable modem if you’re using DSL or fiber, which use entirely different standards from DOCSIS. You also shouldn't buy a cable modem if you use your cable provider for telephone service, bacuase your cable modem may not have phone ports.

The best cable modem for normal usage: Arris SURFboard SB6141 (Compatible with more of the biggest ISPs)

The best cable modem for high-speed plans: Arris SURFboard SB6183 (a bit more pricey)

Tip:

To prevent cross-site-scripting attacks that trick you into rebooting your modem or restoring it to factory settings: Just go into your router’s configuration page and block access to the IP address 192.168.100.1 (the modem’s configuration page) for anyone inside your network. 
Doing this  will prevent you or anyone else on your network from accidentally clicking a link that tricks your modem into rebooting.