Find that program running on that port

macOS (tested on 10.11.6):
sudo lsof -i :80
…where ’80’ is the port you’re curious about (substitute any value).  

Thanks to Databasically. See also here.

Windows (tested on 10 Pro version 1709):
netstat -ano
…this will produce a list of ports in use and the PIDs using them)

Thanks to @RickVanover.

Fedora 20
netstat -npl | grep 8013
…this will find the name of the process running on port 8013. Thanks to Vivek Gite at nixCraft.