(Before you start...You may find the videos at the bottom of this page helpful.)
Below is a list of Comcast/Xfinity approved retail modems and gateways that can be used in lieu of rental equipment. This is not an official list but is compiled and maintained by the community. Please visit https://mydeviceinfo.xfinity.com to verify that a specific device will work with your service. If you find an error or information that needs to be updated, please feel free to send me a private message. Thank you!
Definitions
Modem: This is the basic device you need to connect your home network to Comcast. A modem will provide a single ethernet connection. Most people connect the modem to a router in order to have more than one wired device and WiFi service. Some modems provide voice capabilities.
Gateway: Many people don't want to deal with a separate modem and router. A gateway device is a combination modem and router. They typically allow four wired connections and provide WiFi. Some gateway devices also provide voice capabilities.
Channels: These are communication paths. The more channels the better. This is expressed as downstream channels x upstream channels. So a device that is labeled "32x8" has 32 downstream channels and 8 upstream channels. Higher speed tiers require modems with more channels. Each channel can handle ~25Mbps, therefore an eight channel modem would be capable of ~200Mbps.
To find your speed tier visit https://mydeviceinfo.xfinity.com. After you log into your account, your speed tier will be displayed on the left-hand side of your screen. Alternatively, you can view the speed tier to which you are subscribed here. Sign in and scroll down to the data usage graph area.
The List
Asus CM-32_AC2600 32x8 gateway
Linksys CM3016 16x4 cable modem
Netgear C3000-100NAS 8x4 gateway
Netgear C3700-100NAS 8x4 gateway
Netgear C6250-100NAS 16x4 gateway
Netgear C7000-100NAS 24x8 gateway
Netgear CBR40 32x8 gateway (Orbi)
Netgear CM500-100NAS 16x4 modem
Netgear CMD31T-100NAS 4x4 modem
TP-LINK Archer CR700 16x4 gateway
TP-LINK Archer CR500 16x4 gateway
ZoomTelephonics 5341 4x4 modem
ZoomTelephonics 5341J 8x4 modem
ZoomTelephonics 5345 8x4 modem
ZoomTelephonics 5350 8x4 gateway
ZoomTelephonics 5352 8x4 gateway
ZoomTelephonics 5354 8x4 gateway
ZoomTelephonics 5360 8x4 gateway
ZoomTelephonics 5363 8x4 gateway
ZoomTelephonics 5370 16x4 modem
DOCSIS 3.1 Devices (required for gigabit speeds)
Motorola MB8600 32x8 modem (DOCSIS 3.1)
Arris SB8200 32x8 modem (DOCSIS 3.1)
Netgear CM1000 32x8 modem (DOCSIS 3.1)
Netgear CM1000-100NAS 32x8 modem (DOCSIS 3.1)
Netgear CM1100 32x8 modem (DOCSIS 3.1)
Netgear C7800 32x8 gateway (DOCSIS 3.1)
Netgear CM1150V 32x8 telephony (DOCSIS 3.1)
Voice Capable Devices
Arris SVG2482AC 24x8 gateway telephony
Arris TG862G 8x4 gateway telephony (Same as TG862G-CT, TG862G/CT)
Arris TG862R 8x4 gateway telephony
Arris TM722G 4x4 modem telephony
Arris TM822G 8x4 modem telephony
Arris TM822R 8x4 modem telephony
Arris SBV3202 32X8 modem telephony
Netgear C7100V 24x8 gateway telephony
Netgear CM500V 16x4 modem telephony
Netgear CM1150V 32x8 gateway telephony (DOCSIS 3.1)
Motorola MT7711 24x8 gateway telephony
Last update 1/30/19 @Nerdburg
Thanks to @darkangelic for his tireless efforts to keep this page updated.