Finding a MAC address is, for the most part, simple. However it depends on which operating system you are using. Here are a few tips on how to find your MAC address when your computer runs certain popular operating systems

Windows NT

If you are running Windows NT, finding your MAC address is quite simple. With TCP/IP installed, run the following ipconfig/all within your MS-DOS shell window. The MAC address will be clearly visible.

Windows 2000 and Windows XP

Open up your command prompt window by clicking Start, then Run, then cmd. Once in the command prompt window, type ipconfig/all and the MAC address will be visible. Please note that the 12 digit Physical Address is actually the MAC address.
MAC OSX

On your MAC OSX computer, open the system preferences and select Network. Now you will need to choose an interface. Remember to choose an interface of the MAC address you are trying to locate. You can do this by going to Show and then looking in the drop-down list. For those individuals looking for the MAC address of built in Ethernet cards (NIC), simply select the Ethernet Tab and read the MAC Address which is labeled as "Ethernet ID".

Note: If you have a wireless card, follow the above steps, but instead of selecting the Ethernet card, choose "Air Card". Your MAC address will be labeled as "Air Card ID".
Finding Your MAC Address on a Linux OS

On most Linux systems, the Ethernet device (NIC) is called eth0. To find the MAC address on these devices, first become the root using the su command. Then simply type the command ifconfig -a. You should see a 12 digit address usually labeled HWaddr