Here
It's called Look@Lan.
It will do some basic host detection and interrogation. For example, I configured it with my subnet range and evey 5 minute (10 default) it will go out and ping sweep my network.
Of course, if there is someone out there who is sniffing my traffic and they have not got a valid IP (which they don't need to sniff) then it won't find them. But if some Joe comes into my network range and I'm not running WEP, gets an IP and starts to surf I'll see him.