DjVuLibre is an open source (GPL'ed) implementation of DjVu, including viewers, browser plugins, decoders, simple encoders, and utilities.
(From their web site) DjVuLibre (pronounced like the French "déjà vu libre", which means free DjVu), is an enhanced version of that code maintained by the original inventors of DjVu. It is compatible with version 3.5 of LizardTech's DjVu software suite.
 DjVuLibre includes
- A standalone DjVu viewer for Unix under X11 (based on the Qt library).
- A browser plugin that works with most Unix browsers, including: Netscape-4.x, Netscape-6.x, Mozilla, Galeon, Konqueror, and Opera.
- A full-fledged wavelet-based compressor for pictures.
- A simple compressor for bitonal (black and white) scanned pages.
- A very simple compressor for scanned color pages.
- A compressor for palettized images (a la GIF/PNG).
- A full set of utilities to manipulate and assemble DjVu images and documents.
- A set of decoders to convert DjVu to a number of other formats.
- An up-to-date version of the C++ DjVu Reference Library
Windows and MacOS X versions of the viewer/plug-in, free Windows compressors, and high-end commercial compressors and OCR engines are available from LizardTech Inc. The compressors provided here are slower, produce larger files (sometimes with lower image quality) than the commercial compressors, but they do the job.
For those who like quick solutions without having to install software, a variety of free web-based conversion services are also available, including Any2DjVu and Bib2Web.