Network Meter is a small portable tool for monitoring network traffic.
The program doesn't require installation. To get started, just launch it, click Monitor and select the network adapter to track.
Network Meter displays a real-time graph of network traffic, and can show either the data transferred or the current network speeds.
Unusually, the program can display separate graphs for each of your network adapters. Select as many as you need in the Monitor menu and they'll be displayed one above the other, for easy comparison.
There are a few basic options. Most usefully, right-clicking and selecting "Always visible" keeps the program on top of other windows.
What you don't get is any way to do anything with the collected data, beyond look at the graphs. There are no export options, no way to save any kind of report or even a screenshot of a graph.