If you are trying to identify a network device, and all you have is a MAC address for that device, you might try identifying which hardware vendor the MAC address range is associated with. For example, I had a device connected to my wireless which had a MAC address starting with the prefix 30:69:4B. I could not identify exactly which device it was, although it was most likely a valid one.
The device also did not show up in other tools, such as the Windows command line arp -a command, Angry IP Scanner or Colasoft's MAC Scanner, so identifying it that way was not possible. ARP stands for Address Resolution Protocol, i.e. the protocol used to determine MAC addresses from IP addresses so that transmission at the link layer can occur. You didn't forget your OSI model did you? :)
Using http://hwaddress.com/ I was able to determine the device was a coworker's Blackberry phone which was associating with the wireless access point (AP). The MAC prefix is owned by the manufacturer RIM/Research In Motion. Another similar search site is Vendor/Ethernet/Bluetooth MAC Address Lookup and Search, although I didn't find what I was looking for on that one.