Platform IndependentTo install ImageJ 1.48 on a computer with Java pre-installed, or to upgrade to the latest full distribution (including macros, plugins and LUTs), download ij148.zip (3MB) and extract the ImageJ directory. Use the Help>Update ImageJ command to upgrade to newer versions.
Mac OS XDownload ImageJ 1.48 (4.7MB) 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. An experimental version of ImageJ bundled with Java 8 is also available (requires OS X 10.8 or later). Instructions.
LinuxDownload ImageJ 1.48 bundled with 64-bit Java (40MB) or with 32-bit Java (46MB). Both versions include Java 1.6.0_24 from Oracle. Instructions.
WindowsDownload ImageJ 1.48 bundled with 64-bit Java (25MB; requires 64-bit Windows), 32-bit Java (28MB) or without Java (5MB). Can't use an installer because you lack administrator privileges? ImageJ is also available as a ZIP archive (ij148-jdk6-64.zip or ij148-jdk6-32.zip). Instructions.
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 120,000 lines of code in 350 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.