Web site Linux.com offers a few tips for taking advantage of the second Ethernet port on the back of your computer. For example, in Linux you can bond your two ports for load balancing and fault tolerance.
...bonding [combines]both of the computer's interfaces into a single interface.... The OS can alternate which interface it uses to send traffic, or it can gracefully fail over between them in the event of a problem. You can even use it to balance your traffic between multiple wide area network (WAN) connections, such as DSL and cable, or dialup and your next door neighbor's unsecured Wi-Fi.
Most new computers come packed with a perplexing abundance of Ethernet ports—that is to say, two. But with a little know-how on your Linux box, you can get a whole lot more from them. The article focuses on Linux, but if you've found a use for your second Ethernet port on your Windows or Mac PC, let's hear about it in the comments.