This one is too easy... and therefore has limitations.
The author of this Perl Module Net::MacMap made this impossibly simple. I've attached the sample he provides with his module (see below). Just to clarify, this sample is hard-coded with a sample MAC address. Simply modify this to accept input from STDIN, or via other methods... sniff, NS, etc.
This script is quick and easy, but cannot be easily updated as the author included the OUI data in the module. Anyway, here's the simple script.