Squoosh is one of those tools that immediately makes sense the first time you use it. You drop an image into the browser and the screen splits in two: the original on the left, your compressed version on the right. You can drag a slider between the two to see exactly where quality starts to degrade as you push the compression further. It sounds simple, but the visual, interactive nature of that comparison makes it far more useful than just looking at file size numbers.
The format support is comprehensive. You can compress to MozJPEG, WebP, AVIF, OxiPNG, and several other formats, and the settings for each codec are exposed so you can tweak quality, effort level, and other parameters. I find this particularly useful when working with images destined for the web, since it lets you make an informed judgement about whether the difference between AVIF at quality 65 and quality 75 is visible enough to justify the larger file size. That kind of hands-on experimentation builds genuine intuition for image optimisation.
Being built by Google and open source means you can trust the compression codecs are well-maintained and up to date. The tool runs entirely in the browser using WebAssembly, which means no images are uploaded to a server and processing is fast even for large files. There’s also a command-line interface available if you want to batch-process images as part of a build pipeline. For individual image optimisation and format comparison, Squoosh is the best free tool available. It’s genuinely enjoyable to use, which isn’t something I can say about most compression utilities.