an interpreter for the PostScript language and for PDF.
The leading edge of Ghostscript development is under the GNU Affero GPL license.
Ghostscript releases can be downloaded here.
We have also started collating a Frequently Asked Questions page. It is early days, but may prove useful:
If you want to contribute patches to Ghostscript, GhostPDL or MuPDF you will need to read, understand and sign the Artifex Contributor License Agreement. We also have a bug bounty program if you're looking for a place to start contributing.
Other page description languages:
A user-friendly viewer:
A JBIG2 image decoder: