Char Jackson wrote:

In the old days, you'd stick a hub there, instead of a dual-NIC PC, but hubs
have pretty much disappeared now. I haven't seen one since the mid to late
80's, and it was proudly advertised as 10 megabit, so it'd be a bottleneck
by today's standards.

Nowadays you could use a gigabit switch with port mirroring, this one is
considerably cheaper than an 'enterprise' switch ...

http://www.dual-comm.com/port-mirroring-LAN_switch.htm