LaTeX, Macro package on top of TeX, written by Leslie Lamport, that makes it much easier for most people to produce extremely high-quality typeset TeX documents. Has a huge number of extension packages that allow it to do things from reports to presentations to crosswords.
MC 2005-12-29: I've typeset a couple of books now and find that I get a lot of mileage out of getting my content into an XML format and then writing a Tcl script (making use of tDOM) to transform that into LaTeX. This allows me to easily experiment with LaTeX markup and LaTeX packages (since I've still got a lot to learn) with minimal pain.
joheid 2016-10-10 We use Tcl / LATEX to generate test-protocols from data tests. It is more or less patching in a template. The subst command comes very handy for this task.
GOSH is a tool for converting plain ASCII text to LaTeX and other document formats. Written in Tcl.