Imgur - Imgur is a free and simple image hosting service with image editing feature. Signup is optional.
Free Texture Packer - Free Texture Packer is an open source tool that allows you to pack multiple images into one atlas for you games or sites.
Flickr - image and video hosting website
Sprite Sheet Packer - Sprite Sheet Packer is online that you can easily use to pack sprite sheets, including data formats for CSS, LESS, and Phaser.
Photobucket - Photobucket offers image hosting and free photo and video sharing.
Sprite Monkey - Small and fast but still full-featured sprite sheet editor.