NeonView is a new minimalist, lightweight image viewer, completely free and open-source, licensed under the GPLv2 and developed by TuxArena.
This first release, codenamed ‘Betta splendens’, includes just a handful of features for now, however it is the base on which development of more advanced features will take place. Still, the goal of NeonView is to remain clean and lightweight, while also trying to implement only the needed functions that a simple image viewer should have.
Some of the features of NeonView:
This article overviews five image viewers available for Ubuntu and also includes at the end a list of another five which either are no longer maintained or are based on older libraries (KDE3 for example). Update: Two more viewers have been added, PhotoQt and Berry, increasing the number of viewers to 12.
In my opinion this is probably the best image viewer available at the moment. Built for KDE4, Gwenview comes with support for virtually all image formats out there, tools to do basic editing, tree-like file browser, support for tags, thumbnail previews, cropping, image rating system, slideshow, fullscreen mode, support for plugins and two view modes (Browse and View). An overview is available here.
sudo apt-get install gwenview