XnConvert is an easy image converter for graphic files, photos and images available on Windows, MacOSX and Linux.
GraphicsMagick is the swiss army knife of image processing. Comprised of 282K physical lines (according to David A. Wheelers SLOCCount) of source code in the base package (or 952K including 3rd party libraries) it provides a robust and efficient collection of tools and libraries...
GraphicsMagick vs ImageMagick opinions
command line based and does many of the same functions as ImageMagick. The command line interface is easier to understand then ImageMagick and it is extremely fast.
Gifsicle is a command-line tool for creating, editing, and getting information about GIF images and animations. Making a GIF animation with gifsicle is easy.
Phatch is an user friendly, cross-platform Photo Batch Processor and Exif Renamer with a nice graphical user interface. Phatch handles all popular image formats and can duplicate (sub)folder hierarchies.
Last code commit in 2010, latest release from 2014.
G'MIC is a full-featured open-source framework for image processing, distributed under the CeCILL free software licenses (LGPL-like and/or GPL-compatible). It provides several different user interfaces to convert / manipulate / filter / visualize generic image datasets...
G'MIC vs ImageMagick opinions
It isn't better than Imagemagick, but it's an extremely powerful tool and unlike IM it can be installed into GIMP, making it more powerful than Photoshop.
Converseen is a free cross-platform batch image processor for Windows and Linux that allows you to convert, resize, rotate and flip an infinite number of images with a mouse click.Warning
Installation without activation key works showing different dangerous 3rd party softwares (bloatware, spyware, etc.). Avast Antivirus detected at least 3 different kind of viruses (or potentially viruses/dangerous software).
NConvert is a powerful command line multi-platform batch image processor with more than 80 commands. Compatible with 500 image formats.
The official website is no longer available. Last version, 1.0.1, released in January 2012, can be still downloaded from Wayback Machine.