Windows 2000/XP/7:
Start menu, Run: cmd.exe type in getmac -v
Here you need to look at the columns Physical Address. There may be more than one line. If you are connected to a network via UTP cable you should choose one under Ethernet adapter Local Area Connection (or something similar), or if you use Wi-Fi, then the one following Wireless LAN adapter Wireless Network Connection.

Windows 8/10:
This process is similar to the previous version of Windows, but the command line can be started on the desktop by right-clicking the Start menu icon.

Linux distribution:
Terminal: ifconfig
Here, lines with HWaddr contains the relevant data.

OS X:
Terminal: networksetup -listallhardwareports
The MAC address is displayed in the line starting with Ethernet Address.