One of the better uses for a spare set of NICs (note that I said set) is to create a secondary link between two computers. The secondary link can use just a crossover cable (or straight through with some NICs). The secondary link avoids whatever conjestion is on the first, and if just a link between two computers, can always work in full-duplex mode. Of course if the primary link is through a switch, there isn't that much benefit to this, unless if the primary nic is busy responding to other hosts.
I recently did this with gigabit between my main workstation and another. I liked it so well that I re-did my server (and bought a bunch more gigabit nics to do this).
I now have a fully switched 10/100 network for everything, and in addition, three seperate gigabit links to my server from 3 workstations. With a bit of wizardry, that also means gigabit speed between those three workstations (IP Forwarding in the server).
I have some specs in a thread here:
http://forums.anandtech.com/messageview.cfm?catid=36&threadid=656424
Only downfall to something like this is the wiring gets crazy, you have to setup all your routes custom on each machine, as well as an lmhosts file, and if you ever turn off your machines, the other one alerts you that the network cable is unplugged (in Win2K) or whatever. Plus you better know how IP gets routed on your machine (know the route command?) before you start, or you'll just have two unused NICs or a non-functional network.
Actually, if you are crazy enough (and understand routing well enogh), you can setup seperate receive/send paths. Ie - send on one NIC and receive on the other. And that would work with just one extra NIC. However, you would probably need to setup custom routes and lmhosts on every machine to make it work consistantly.
As far as Adapter Teaming, it only works with server NICs. Such as the Intel Pro/100 S Server Adaptors. There is no difference between the server and desktop adaptors from Intel, but the Intel Proset II software does recongnize the difference and only lets you make adaptor teams and some other features with Intel server adaptors. I think 3COM has similar features for their line of server adaptors.