imgult is a fast and free solution to lossless image optimization. It is made for Linux and Mac systems.
Is there a GUI?
For Mac users, you can run this script that will install
imgult and all its dependencies, along with the automator version of
curl https://gist.githubusercontent.com/ryanpcmcquen/2cb42266052a57992d55/raw/install-right-click-imgult-mac.sh | sh
imgult will show up in your right-click contextual menu, and you can even set it to a keyboard shortcut! For Linux there is no GUI version yet (but I prefer the command line version, even on my Mac).
For more details on install and usage, check the GitHub repo.