I have 3 Macs, 1 iPad, and 1 Apple TV (using HD Netflix, mostly).
I need to keep an eye on bandwidth usage. What is the best way to monitor the bandwidth usage (preferably per device) that passes through my Airport Extreme router?
The Airport Express/Extreme allows monitoring via SNMP if you enable it via the Airport Configuration Utility (version 5.6 ONLY).
Apple removed advanced configuration parameters from Airport Configuration Utility 6.0. Please download a version 5.6 of the utility from Apple’s support page to get a full access to all configuration parameters of your router. SNMP configuration is located under Advanced settings.
From there, ANY snmp polling engine can get traffic as a whole.
You would be hard-pressed to find anything that could differenciate traffic between devices that isn't a real router (Cisco, etc) or custom built Linux box.