Uppy - The next open source file uploader for web browsers
ImageKit.io - Instant multi-platform image optimization
Uploadcare - File uploading, media processing & content delivery for modern web apps
Teeny Tokyo - A surprisingly-powerful bulk image resizing app
Filestack - Simple file uploader and robust APIs for uploading, transforming, and delivering any file into your app. Filestack is a collection of tools and powerful APIs that make it simple to upload, transform, and deliver content.
Promo Instant Image Resizer - Automatically optimize images for web & social media (free!)