ImageOptim

ImageOptim does one job. You drag a folder of PNGs and JPEGs onto it, and it rewrites them smaller in place, with no visible quality loss unless you ask for one. Under the hood it is a well-chosen stack of open-source encoders — Zopfli, PNGOUT, MozJPEG, pngquant — run in an order that squeezes out more than any of them alone, plus the unglamorous but valuable step of deleting the colour profiles, thumbnails, GPS tags and editor cruft that bloat exported files.

For anyone who ships images to a website by hand, this quietly removes a recurring bill. The upload-a-file compression services exist mostly because people do not know a local tool this good is free.

Two boundaries are worth naming. The desktop app is free; the ImageOptim API, which does the same work on a server or in CI, is the developer's paid product, and that is a fair trade. And the app is a compressor, not an editor — it will not resize your images for retina, convert anything to WebP, or rename files.

If your images already go through an automated build step, or through a CDN that optimises on the fly, you do not need this. It earns its place when the last step before upload is a human with a folder.