Image Resizer for Windows - Lets you resize images by right-clicking.
ImageKit.io - Instant multi-platform image optimization
Squash 2 for Mac - The easiest way to compress and optimize images for the web
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Piio - Generates responsive images so you don't have to optimize
punchi.me - Edit one or several images in a few clics.