network tap

There are a number of vendors that sell those, and I have personally used TAPs from those vendors:

1. Netoptics - www.netoptics.com
2. Datacom - www.datacomsystems.com
3. Finisar - www.finisar.com, not sure if they still sell taps, doesn't look like they do

Then there are others, which I haven't tested myself:

1. Gigamon - www.gigamon.com
2. VSS - www.vssmonitoring.com

and probably a couple more I haven't even heard of yet. I think most taps just work fine, I usually use the very basic full duplex taps from Netoptics and Datacom. My rules of thumb in using taps:

1. Test them before going into the field. You need to be sure they deliver correct results
2. Careful with link aggregation taps. I had two encounters with that kind of tap (once Datacom, once Netoptics) and they didn't work as expected. The Datacom tap put only about 10% of the real traffic on the output lines toward the analyzer, and the link was not even close to being half full (can't remember the model). Netoptics introduced CRC errors into a direct connection between a Cisco Switch and a Cisco router (it was an "iTap Triple-speed Port Aggregator" for Gigabit copper).
3. Add-On Rule to #2: network taps are like modern cars: the less electronic doing fancy things, the better - there's less to break or go wrong :-)