ImageOptim is great software that exists in 2 formats: a free mac app and a webservice.
Shrink.sh allows you to automatically process any image committed to Github. Once the Github App is installed on an organization, it can optimize images added to any of its repositories. There is no need to run a desktop app or maintain manual processes for each repository.
The ImageOptim webservice is ideal if you want to optimize user-uploaded images before storing them in the cloud. It isn't an ideal option if you are trying to optimize images in other cases or if you are processing fewer than thousands of images per month.
The Mac App is great when you need to optimize a handful of images one time, but it is a tedious and often overlooked manual process. When working with a team, everyone must be taught and reminded to use the tool.
Imgix is a CDN that specializes in image optimization.
Shrink.sh is designed to optimize images committed into your source control repository. Shrink.sh is great to ensure all of the images that get added go through an optimization review step. Shrink.sh also has a very different pricing model. Imgix serves your images from their servers and charges for the bandwidth used, while Shrink.sh optimizes the images your server is serving and charges only a flat monthly rate.
Imgix is the perfect choice when you need to dynamically optimize images. If you have thousands of images or user-uploaded images and need to deliver them in many different sizes, it will really shine.