Maybe this is obvious - but after spending 5, maybe 6 minutes on the snort.org page I could not find an explanation. How does the computer hosting the snort.org application "plug into" the network it is monitoring??
I say this because, well, most people use Switches these days. So, any computer hosting snort.org is only going to see packets ment for that computer or broadcast packtets ment for all computers. Any attack specifically targeting another computer would never been seen - the way I understand it.
Given this, I can only think of 1 solution to the OP's question. And that would be to run something like snort.org on the actual switch. I'm thinking the next stop going down this path is to install one of the open source switch/router software packages (like OpenWRT) on an appropriate switch/router box and use it for all traffic in and out of the house. Now you have a box that can see everything and in theory monitor all packets no matter where they are going.
Edit (added later)...
Ug, after all that and now I see you were only interested in the wireless part. And the utility you pointed to is really only listing the connected devices. I don't see any indication that it is looking for malicious behavior:
Wireless Network Watcher is a small utility that scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Ok, so, do you have an Android phone? If you do, go find "Fing" in the app store and download it. Connect your phone to your WIFI and run Fing. It should go out, search for and list all connected devices. Not only the WIFI but also the LAN connected devices.
As for the Arduion, the Atmel processor used in an Arduino Uno is generally not powerful enough to handle all the Ethernet overhead. That's why the Arduino Ethernet shield is so expensive (WRT other shields). It likely has it's own more powerful processor. But if all you are wanting to do is "ping" every possible address (256 of them) on a subnet like "Fling" or "Wireless Network Watcher" ... I believe this is possible. See, this is why we have not been discussing the Arduino - because an Arduino will likely not be able to work fast enough to catch malicious attacks. But pinging address can be done slowly and an Arduino should be able to keep up with that.