Granet

Granet (Graph networking) is a simple, no-cost utility that graphs the throughput of each active network interface on your computer.

It works with most any network interface recognized by the OS (including Ethernet, Airport, and PPP). Each interface is displayed in its own window which can be moved and re-sized from small to large. Each window scales independently so the graph is drawn appropriately throughout the throughput range. What's New
Version 1.3 fixes a problem that could prevent the proper detection of dial up (PPP) interfaces. Requirements
PPC, Mac OS X 10.1 or later