An open-source, web-based 'multi-up' viewer that supports zoom-pan-rotate functionality, ability to display/compare simple images, and images with annotations.
Wind Viewer is a level viewer and editor for Zelda: The Wind Waker and (to an extend) Twilight Princess.
Jappix Me, the very first XMPP public profile viewer. Support for Microblog, Comments, vCard, Avatars & Bio.
iOS App for GitHub that is viewer of repositories and source code.