Gwenview
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).

sudo apt-get install gwenview

Eye of GNOME
Eye of GNOME is the default image viewer that comes with the GNOME desktop environment, with support for a lot of formats (including JPG, PNG, BMP, GIF, SVG, TGA, TIFF or XPM), with a simple interface and features like slideshow, zooming, fullscreen mode, thumbnail previews, automatic orientation and support for plugins.

sudo apt-get install eog

gThumb
Another GTK viewer, gThumb has some cool features, most of which come as extensions which can be enabled or disabled on demand. Notable features are importing from Picasa or Flickr and exporting to Facebook, Flickr, Photobucker, Picasa or local folder, thumbnails preview, slideshow support, tree file browser, bookmarks, filters, support for extensions.

sudo apt-get install gthumb

Viewnior
This is a minimalist image viewer written in GTK+ with basic features such as image rotating/resizing, fullscreen mode, save as JPG or PNG, image properties, slideshow and cropping images.

sudo apt-get install viewnior

gPicView
gPicView is the default image viewer in LXDE, with a compact interface (the toolbar is located at the bottom of the window, menubar is nonexistent and all the functions are available by right-clicking the image space). gPicView features fullscreen mode, rotating/flipping, and saving images as JPG, TIFF, BMP, PNG and ICO.

sudo apt-get install gpicview

5 More

I also have to mention here some image viewers which don’t seem to be maintained anymore (some had their latest release years ago, some are only available for KDE3 etc). Included here are:

KSquirrel – This is a KDE3 feature-rich image viewer with support for thumbnails, a tree-view file browser, support for KIPI Plugins and usual functions for rotating or zooming images.

QIV – Or the Quick Image Viewer, this one has a basic interface and comes with features like image zooming and scaling, gamma, contrast and brightness correction and slideshow.

GImageView – Although this has not been updated since 2004, GImageView uses GTK+ and can still be used as a decent viewer. It comes with file browser, thumbnail previews, tab support, rotating/zooming images, being highly configurable in the same time.

GQview – Yet another GTK+ image viewer, GQview has a clean interface and includes a file browser and decent features.

KuickShow – This is KDE3-based, and comes with a file manager, basic image manipulation functions and a handful of configuration options.