I'm trying to keep tabs on my home wireless network. Is there a good, GUI based tool that can show connected machines (make sure I don't have 'guests'), traffic stats, etc. Thanks.
Intermapper - a commercial product that has a five license version for free. http://www.intermapper.com/products/free-tools/five-for-free. Use snmp to pull traffic details from your router or netflow if your router supports it.
As well as snmp, Intermapper has lots of probes for standard services and even specific devices. They even have a growing library of user contributed probes to monitor things link UPSes and environmental monitoring devices.
Network Magic has a free trial. I use it to keep tabs on my PC's and people connecting. It also keeps track of game systems and well, pretty much everything that connects to your Wi-Fi network.
I was able to get Zabbix running on OSX.
Although it's not specifically tailored for monitoring OS X, it can monitor any unixy OS pretty well. I've personally used it to monitor Xserves, Minis, iMacs, MBPs, and even an Airport Express (in addition to a plethora of non-apple stuff, of course)
Disclaimer: I'm the Zenoss Community Manager
Zenoss is a free and open source network monitoring tool, with an OSX installer. We've got a number of OSX users, monitoring everything from desktops to XSans to Airport Extremes. I monitor my Airport Express and Extreme and a couple of laptops with it and I've been working on SSH-monitoring in addition to the existing SNMP.