ImageJ is a Java open source image processing program designed for scientific multidimensional images. It is highly extensible, with thousands of plugins and macros for performing a wide variety of tasks, and a strong, established user base.

There are three major versions of ImageJ:

ImageJ1 - The stable version, developed by Wayne Rasband at NIH since 1997

ImageJ2 - Focuses on analysis of scientific multidimensional image data. Includes ImageJ1 with a compatibility layer

Fiji - A distribution of ImageJ for the life sciences. It provides a large number of additional plugins to facilitate analysis of life sciences images, particularly microscopy images. Built on the ImageJ2 platform.