Tracemac

freshmeat.net: Project details for Tracemac
"Tracemac is a command-line C++ application that traces a MAC address through the bridging tables of Cisco switches. Unlike some other programs, it uses SNMP. It is not necessary to configure it with network structure information, and no switch passwords need be divulged."
Homepage:
http://www.thtech.net/products/6

There is no Windows version of tracemac, but if you've got access to a Linux box this is a truly awesome Layer 2 network diagnostic utility. I had trouble resolving all the dependencies so Brady Alleman (author) was kind enough to compile a static version that runs on Fedora (and probably others).
On a Cisco network, tracemac can be used to quickly locate a machine by MAC address which we find particularly useful for assigning the proper VLAN when a machine has been moved to a new, possibly undisclosed, location. It's also great for locating machines that may have 'gone missing' from their owners - imagine being the person who's using a stolen laptop when the police come knocking on their door.