Ubuntu Internet Connection Sharing

This is one of those occasions for the Staple's "That was easy" button.

Right-click on the network icon
Edit connections
Select Eth[your secondary port]
Edit
Change the name to ICS or whatever to help you remember
Click the IPV4 tab
Method: Shared to other computers.

By default it does NAT, not bridging, so your connected computer will get a 10.x.x.x address, not a 192.168.x.x.

It beats setting up a dd-wrt as an ethernet bridge by a small margin. However, that doesn't look too difficult either. I just didn't have the time to fiddle with it.