Over the years I have had the good fortune to have a number of very hard working and talented people working with me. Aaron Lewis at ADL Datacomm has worked with me in various capacities for almost 10 years now.
He's just written an interesting HOW-TO describing how to create a transparent ethernet bridge with a PC, a couple of network cards and Linux. You can read it over at the ADL Datacomm web site.
Such a bridge allows systems adminstrators to put a machine on a network without an IP address, and to read all the traffic passing through it. The lack of an address makes it more difficult to remotely compromise and by reading the traffic administrators can spot "signatures" or patterns in the data flow that indicate an attack is in progress. Such monitoring is the basis for a lot of intrusion detection systems (IDS).
Bridges are also used in older networks to join networks segments that have different physical network protocols; such as making token ring talk to ethernet segements.