I use Netuse Traffic Monitor. It uses the the SNMP function of your router to show traffic. The dev. has a free utility you can run to make sure your router supports SNMP before you buy the app.
I was using this with the previous gen. Time Capsule and it worked perfectly. Unfortunately, Apple removed SNMP support from the new tower shaped Time Capsule I bought, so the app no longer works for me. But if your router supports SNMP it will work.