Platform IndependentTo install ImageJ 1.45 on a computer with Java pre-installed, or to upgrade to the latest full distribution (including macros, plugins and LUTs), download ij145.zip (3.1MB) and extract the ImageJ directory. Use the Help>Update ImageJ command to upgrade to later versions.
Mac OS XDownload ImageJ 1.45 (5.5MB) as a double-clickable Mac OS X application. Includes ImageJ64, which uses Java 1.6 in 64-bit mode on Intel Macs running OS X 10.5 or later. (Instructions)
LinuxDownload ImageJ 1.45 bundled with 32-bit Java (46MB) or with 64-bit Java (40MB). Both versions include Java 1.6.0_20 from Sun and the ImageJ source code. (Instructions)
WindowsDownload ImageJ 1.45 bundled with 32-bit Java 1.6.0_20 (28MB), with 64-bit Java 1.6.0_20 (24MB; requires 64-bit Windows) or without Java (3MB). (Instructions)
DocumentationTiago Ferreira's comprehensive
The ImageJ User Guide is available as an 8MB PDF document. The JavaDoc API documentation (1.9MB ZIP archive) is also available.
Source CodeThe ImageJ Java source consists of 92,000 lines of code in 265 files. It is available online and as a 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.