nmap -sP 192.168.1.0/24

Starting Nmap 5.21 ( http://nmap.org ) at 2011-07-12 17:37 CDT
Nmap scan report for 192.168.1.1
Host is up (0.00036s latency).
MAC Address: C0:3F:0E:D8:C6:E6 (Unknown)
Nmap scan report for 192.168.1.100
Host is up (0.000090s latency).
MAC Address: 00:17:31:EF:F9:51 (Asustek Computer)
Nmap scan report for 192.168.1.101
Host is up (0.0070s latency).
MAC Address: 60:33:4B:2A:38:51 (Unknown)
Nmap scan report for 192.168.1.119
Host is up.
Nmap done: 256 IP addresses (4 hosts up) scanned in 4.21 seconds