Sniffing gigabit ethernet? 1000baseT LAN taps?

[Joshua Judson Rosen]

Trying to debug some comms problems between a couple of 1000baseT
ethernet devices with poor visibility; can anyone recommend any
inexpensive tap mechanisms that will let me run tcpdump or the like on
the link from a Linux laptop? I'm actually not quite sure what my exact
threshold for "inexpensive" is on this; I just don't have a 3-port
GigE Linux box on hand right now, and don't know that I want to
bother trying to build/buy one in the timeframe I'm working in....

When this problem came to me, I remembered having seen these little LAN tap
things:

        http://hakshop.myshopify.com/products/throwing-star-lan-tap-pro

... but I'm not sure one of those would actually work for me: either of
the nodes I'm sniffing may not actually fall back to 100baseT
gracefully, and I'd like to be able to sniff the traffic even if the
devices are operating in full duplex mode.

I've found various other devices that look more like this:

     http://profitap.com/C1AP1G-0

Two gigabit ethernet ports with a *USB* cable? I'm not sure what they're
even doing with a setup like that; wondering if it's something that
I'd actually need their special Windows-only software to actually
use....

-- 
"'tis an ill wind that blows no minds."