Hover Zoom - Enlarge thumbnails on mouse over.
Image Max URL - Help find larger or original versions of images, including a customizable image popup feature
Hover Zoom+ - Google Chrome extension for zooming images on mouse hover - extesy/hoverzoom
ImagePreviewer - This extension shows a popup with an image preview when hovering over link that points to an image file or a page with an image.
Hover for Safari - Hover for Safari allows you to expand any image, link, or GIF just by hovering over it.
Reddit Enhancement Suite - The Definitive Reddit Add-on