AnalogX AnonyMac

AnalogX AnonyMac allows you to quickly see all of your network connections, what MAC is currently assigned to them, and then change them to a completely new one.

The program also decodes the manufacturer information from the MAC so you can see who actually made the NIC, as well as some of the basic IP settings associated with the connection. There is also an ARP viewer, which gives you the ability to see any of the MAC addresses of other computers on your local network along with decoded viewing of their MAC manufacturer.
AnonyMac works on all versions of Windows, from Window 95 to Windows 7 and everything inbetween (including XP, Vista, Win2k, etc).