Reddit Enhancement Suite - The Definitive Reddit Add-on
Hover Zoom - Enlarge thumbnails on mouse over.
Pager - Doctor house calls on demand
Image Max URL - Help find larger or original versions of images, including a customizable image popup feature
Moderator Toolbox for Reddit - An extension that adds a myriad of much needed features and functionality to reddit for moderators.
Hover Zoom+ - Google Chrome extension for zooming images on mouse hover - extesy/hoverzoom