Aspic is a program that generates line art images from a text description of a picture that contains commands such as "line", "box", "circle", and "arc". Aspic's concept is similar to the "pic" command. Output is either encapsulated PostScript, or Scalable Vector Graphics (SVG). Quite complex pictures can be constructed from Aspic's primitives, which include facilities for positioning text alongside graphics, and the use of colour.
Philip's Music Writer is a program for typesetting music. It reads text files as input, and generates PostScript as output. It can also write simple MIDI files for proofhearing purposes. PMW is written in C and is freestanding; that is, it does not require additional processing software. It is a Linux/Unix port of a program that has run for over a decade on Acorn systems, where it was known as Philip's Music Scribe.
SDoP (Simple DocBook Processor) reads a DocBook XML file, processes it into typeset pages, and outputs the result as PostScript (which can easily be converted to a PDF). It is "simple" because it supports only a subset of DocBook, and also because it does not make use of a DTD or stylesheets or any other heavyweight apparatus. It is a single program. SDoP is used to format the Exim reference manual.
xfpt is a program that reads a file of plain text that contains relatively simple markup, and outputs an XML file. It is intended to simplify the management of XML data. It is not a program that attempts to turn a plain text document into XML. Markup within text is introduced by ampersand characters, but is otherwise "soft". You can define what follows the ampersand, for example, &" to generate a "quote" element. There is also a macro facility that allows for higher level concepts such as chapters, displays, tables, etc.