Linux
On Linux systems, the ethernet device is typically called eth0. In order to find the MAC address of the ethernet device, you must first become root, through the use of su. Then, type ifconfig -a and look up the relevant info. For example:
# ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:60:08:C4:99:AA
inet addr:131.225.84.67 Bcast:131.225.87.255 Mask:255.255.248.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:15647904 errors:0 dropped:0 overruns:0
TX packets:69559 errors:0 dropped:0 overruns:0
Interrupt:10 Base address:0x300
The MAC address is the HWaddr listed on the first line. In the case of this machine, it is 00:60:08:C4:99:AA.