Macro package on top of TeX, written by Leslie Lamport (initially), 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.
Useful links:
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.
Donald Arseneau is quite active on both comp.lang.tcl and comp.text.tex newsgroups.
See also calling pdflatex