Rubbernet
rubbernetapp.com
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Rubbernet provides real-time monitoring so you can keep an eye on the exact state of your live network connections. With the Activity sidebar, you'll be able to see which applications are currently communicating over your network. When connections are idle, they will dim to let you know that traffic has stopped.