Fiji is a distribution of ImageJ which includes many useful plugins contributed by the community.
ImageJ is public domain open source software. An ImageJ user has the four essential freedoms defined by the Richard Stallman in 1986:
- The freedom to run the program, for any purpose.
- The freedom to study how the program works, and change it to make it do what you wish.
- The freedom to redistribute copies so you can help your neighbor.
- The freedom to improve the program, and release your improvements to the public, so that the whole community benefits.