SageThumbs - SageThumbs is a powerful shell extension allowing to preview enormous amount of image formats...
ImageKit.io - Instant multi-platform image optimization
Context Viewer - Context Viewer is an explorer extension in the form of a Control Panel applet that lets you preview...
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Pictus - Pictus is a fast, free image viewer that enables Windows to see thumbnails of supported formats, such as PSD. Give it a spin today!
Piio - Generates responsive images so you don't have to optimize