ImageJ 1.x, often shortened to ImageJ1 or IJ1, is a stable version of ImageJ which has been under continuous development since 1997. It has always been, and continues to be, a one-developer project of Rasband .
ImageJ was originally developed in 1997 as a cross-platform version of NIH Image. ImageJ grew organically over time as Rasband continued to add features according to user requests. Now there are many hundreds, probably thousands, of plugins written by members of a diverse community.
The current version of ImageJ is often referred to as ImageJ2, to differentiate it from ImageJ 1.x. The ImageJ2 distribution actually includes the latest version of ImageJ1 as well, along with a legacy layer for backwards compatibility, which transparently converts between IJ1 and IJ2 data structures as needed.
Schneider C. A., Rasband W. S., Eliceiri K. W., “NIH Image to ImageJ: 25 years of image analysis”, Nature methods 9(7): 671-675 . .