Platform IndependentTo install ImageJ on a computer with Java pre-installed, or to upgrade to the latest full distribution (including macros, plugins and LUTs), download ij150.zip (4MB) and extract the ImageJ directory. Use the Help>Update ImageJ command to upgrade to newer versions.
Mac OS XDownload imageJ (6MB) as a double-clickable Mac OS X application. Also available bundled with Java 1.8.0_91 (requires OS X 10.8 or later). Instructions.
LinuxDownload ImageJ bundled with Java 1.8.0_77 (62MB). Instructions.
WindowsDownload ImageJ bundled with 64-bit Java 1.6.0_24 (37MB), bundled with 32-bit Java 1.6.0_24 (44MB) or bundled with 64-bit Java 1.8.0_77 (70MB).
DocumentationTiago Ferreira's comprehensive ImageJ User Guide is available as an 8MB PDF document and as a ZIP archive. The online JavaDoc API documentation is also available as a ZIP archive.
Source CodeThe ImageJ Java source consists of 132,000 lines of code in 348 files. It is available online and as zip archives.
Example Images31 downloadable sample images and stacks are available in ImageJ's File>Open Samples submenu. These images, and more, are also available as a 8.2MB zip archive.