IP Scanner for Mac

From 10base-t Interactive:

IP Scanner for the Macintosh displays all machines, users & IP numbers of Macintosh users on your local network. It also detects some non-Macintosh devices, and in many cases can even identify the manufacturer of the device. IP Scanner uses Bonjour (the technology formerly known as Rendezvous) to collect realtime user information as well as IP data from Macintosh machines on a local area network and sorts everything in tidy columns, each machine's IP number displayed next to its name (along with the current username of the person logged in to that machine). For small network admins, this means no more shouting around the office to find out who has which IP or, conversely, who is sitting at which workstation in a fixed IP network.