Utility for Visualizing Network Traffic via SNMP
Unfortunately I'm not aware of any serious competition to Cacti, but have you considered using a Cacti virtual appliance? There are several pre-configured VMs available with everything pre-installed and ready to go.
Cacti 0.8.7g
rPath Cacti Appliance
You might also want to take a look here:
A Half-Dozen or So Essential Open Source Network Administraton Tools
and in particular at:
Graph Network Performance with NRG - Part 1
He also has a page on how to install Cacti on OS X.
Now I can't say that this meets your desire for something as simple as a typical OS X application install but it certainly meets the free requirement.
I can't say I've played with any of these tools on OS X bar Mac Ports which I consider an essential on any OS X system.

Another really simplistic app (OS X widget actually) is iEyeNet, it definitely meets the free and simple criteria but isn't anywhere as configurable as Cacti. I tried it with my Airport Extreme and it "just worked".

Another option is documented here, it uses a combination of MRTG/Lingon/Geektool to generate and display the network graphs. Reasonably easy to setup...