Spoof MAC Address in Any OS

How to Change MAC Address in Mac OS X
Since Mac OS X 10.4.x (Darwin 8.x) onwards, the MAC address of wired Ethernet interface can be altered in Apple Mac OS X in a fashion similar to the Linux and FreeBSD methods. To do so, type the following command in Terminal.app:
sudo ifconfig en0 ether aa:bb:cc:dd:ee:ff
or
sudo ifconfig en0 lladdr aa:bb:cc:dd:ee:ff (for Mac OS X 10.5 Leopard)
where en0 is the network interface (numbered from en0, en1, en2 …) and aa:bb:cc:dd:ee:ff is the desired MAC address in hex notation. 
Alternative: Third party tools and utilities to change the MAC address in Windows operating system are plenty, for example: SMAC (direct download link to smac20_setup.ex, supports Windows Vista, XP, 2003, 2000), Macshift (direct download link to macshift.zip, for Windows XP), BMAC (almost identical SMAC MAC changer clone by moorer-software.com), Mac MakeUp(direct download link to macmakeup.zip, for Windows 2000/XP/2003/Vista), MadMACs (MAC Address Spoofing And Host Name Randomizing App For Windows, directly downloadMadMACs.zip), EtherChange (direct download link to etherchange.exe), and Technitium MAC Address Changer (for Windows 2000 / XP / Server 2003 / Vista / Server 2008)