imgix
imgix.js is a javascript library that allows you to easily use the imgix API to make images on your site or app responsive to device size and pixel density. imgix.js allows for intuitive use of imgix features such as text formatting, color palette extraction, color adjustments, effects, and watermarking. imgix.js requires an imgix account to use your own images. Sign up at imgix.com
Pak Cheong
Create thumbnails from images proportionally. This is also a replacement for CSS3's background-size in older browsers in IE6, IE7 and IE8.
Cloudinary
Cloudinary streamlines your web application’s image manipulation needs. Cloudinary's cloud-based servers automate image uploading, resizing, cropping, optimizing, sprite generation and more. Cloudinary's jQuery plugin allows direct uploading from the browser to the cloud and dynamic cloud-based image transformations and effects.