ImageJ is an all-in-one image processing tool, able to view, edit, convert and analyze a wide range of graphic formats. The utility is quite lightweight considering the impressive number of capabilities; this however comes at the cost of the graphic interface as ImageJ does not have the layout that most editors employ and is targeted at power users.
With this application you will be able to automate editing tasks by recording commands and then using macros to implement them. Support for plug-ins adds that extra extensibility, which users look for (there are currently over 500 plug-ins available that cover filters, acquisition, segmentation and many more categories).
The utility works with multiple types of image extensions like GIF (animated), TIFF, PNG, BMP, JPEG, RAW as well as video formats (AVI).
In terms of graphic processing and analysis, ImageJ stands as a very solid tool; you can add image effects, detect particles, summarize, view the histogram. One important feature is the ability to perform the same action (measure, convert) on multiple images at the same time.
ImageJ may not be the easiest tool to work with for novice users (functions are not very intuitively dispersed along the interface and the toolbar is not the best way to access them), but this is definitely the application of choice for advanced ones; you can thoroughly analyze just about any image format, draw, convert between different extensions, capture the screen and so much more.