ImageKit.io - Instant multi-platform image optimization
SageThumbs - SageThumbs is a powerful shell extension allowing to preview enormous amount of image formats...
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
MysticThumbs - MysticThumbs generates thumbnails of many image types not natively supported by Windows.
Piio - Generates responsive images so you don't have to optimize
Icaros - Icaros is a collection of lightweight, high quality, Windows Shell Extensions