ImageKit.io - Instant multi-platform image optimization
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Promo Instant Image Resizer - Automatically optimize images for web & social media (free!)
Piio - Generates responsive images so you don't have to optimize
Promo.com - Every promotion starts with Promo.com.
Teeny Tokyo - A surprisingly-powerful bulk image resizing app