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...

I know this is an old topic and also I'm aware of how sensitive recommending one's own application can be (especially in case of a commercial application), however I think this does answer the OP's question exactly. If this is not allowed (I tried looking for a policy in the FAQ), please let me know and I will take this down.

I've been looking for such application for OSX myself. I couldn't really find one (except for the afore mentioned iEyeNet, which didn't meet my requirements) and so I've decided to write one myself, which gave me the opportunity to learn a bit of Objective-C.

I call it iNetGrapher. I think it will meet your requirements although it's not free. It's very simple to install (essentially get it on Mac App Store), doesn't require a web server or any other service / daemon / dependent software, it does autodiscovery of all interfaces on a supported network device / server and it lets you simultaneously graph traffic from multiple interfaces and multiple devices. It supports SNMPv1 and SNMPv2c, and in the latter case, both 32-bit and 64-bit counters.

There is also a free, good alternative, although it is command line only - it might fit your needs though. It's called ttg - you'll need to open this page and scroll down till you find it, or download the source from here: ttg.c. This one has been written by a good friend of mine.

I hope this is useful. I've been genuinely trying to help here and not promote my app.