http://www.cs.wisc.edu/~ghost/index.html
GNUish interpreter for a language very similar to PostScript.
It's actually supposed to be an exact PostScript implementation. You can use it to print your files on your non-PostScript-capable printer, but a cooler approach is to use it as a more-or-less ordinary programming language. If you start it with `gs', it will show a prompt at which you can enter PostScript commands, and a window in which you see the result of drawing commands.
Written by PeterDeutsch, who invented JustInTimeCompilation and applied it to the SmalltalkLanguage in 1983, producing the first implementation of Smalltalk that could run fast enough on cheap hardware. Peter could very well be the best VirtualMachine implementor in the world. So, it is not surprising that he wrote a good PostScript implementation.
These days, GhostScript is much more than simply a PostScript interpreter. It also handles PortableDocumentFormat files, and does a number of interesting bits of TransCoding?, including PostScript to PDF, which is essentially the function of Adobe's Acrobat Distiller product.
That said, GhostScript has a number of serious weaknesses, including generally poor UserInterface, and also a messy, uneven collection of drivers for printers. I'm really hoping to see things improve on both counts. -- RaphLevien
Raph didn't mention that he took over the maintainership of GhostScript: http://www.artofcode.com/ghostscript/openletter.html BrokenLink 20090509 which is not so surprising as this was last edited in 2003.
GhostScript has since gone through many releases.