Here is an easy way to find what machines are running on you local network.

$ nmap -sP 192.168.1.0/24

Starting Nmap 4.53 ( http://insecure.org ) at 2008-04-20 00:06 EDT
Host prime.TetraSys.org (192.168.1.1) appears to be up.
Host borboleta.TetraSys.org (192.168.1.3) appears to be up.
Host 192.168.1.100 appears to be up.
Nmap done: 256 IP addresses (3 hosts up) scanned in 14.148 seconds