The free plugin that started it all, the EWWW Image Optimizer can use freely available optimization tools directly on your web server OR use the Compress API to compress & resize JPG, PNG, and GIF images as well as optimize SVG & PDF files. Available on GitHub and WordPress.org.
Now you can get all the speed boosting tools that I use personally, in one handy package. Includes page caching, JS/CSS optimization tools, ability to disable unused JS/CSS, Google Font optimizer, CDN rewriter, and more.
A plugin for WordPress that lets you optimize images in Amazon S3 buckets, and integrates with the EWWW I.O. plugins to provide full compression capability.
The Standalone Image Optimizer includes all the functionality of the WordPress plugins (local tools + API capability), but with minimal requirements, and no dependencies on WordPress. Read more about it at GitHub.
A WordPress plugin that automatically resizes huge image uploads. Are contributors uploading huge photos? Tired of manually scaling? Imsanity to the rescue!