Squash 2 for Mac - The easiest way to compress and optimize images for the web
Optimage - Advanced image optimization tool
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Aerate for macOS - Ridiculously good lossless image compression
Image Resizer for Windows - Lets you resize images by right-clicking.
Compare to Dev - Image comparison tool