WiFi Security Guard is a free command-line tool to monitor and shield your wireless network from hackers and unauthorized users.
It's swift ARP based multi-threaded scan helps you to perform a quick scan of your entire network in few seconds. You can either do an automatic scan of your local network or manual scan of selected portion of the network. For each discovered system it displays following information:
Host IP Address
Host MAC Address
Along with the above details, it also automatically identifies your local computer or router/modem among discovered devices.
Being a command-line tool makes it easy for automation. If you are looking for a GUI version, check out WiFi Network Monitor.