Lout is a typesetting system. It was designed by Jeffrey H. Kingston, in Basser Department of Computer Science in the University of Sydney. The reference C implementation of the language is called "Basser Lout", and runs on a few notable systems (such as most modern *NIXes), and is distributed under GPL license. The origins of the language lie in mid-1984 and the first public release was in 1991. Currently, Basser Lout is at version 3.28.
In many ways, Lout is similar to TeX and LaTeX languages, excels in similar fields, and it operates fairly similarly - a source file with formatting commands is read by the interpreter that then produces a PostScript output (the interpreter also supports some other output formats like plain text and PDF, even when the PDF support is very limited at the moment - but there's always GhostScript's ps2pdf!)
The actual language is quite a different beast than TeX, however. While the concepts seem quite similar to LaTeX, the actual language may look a bit odd. A TeXnician may guess that @B{this is bolded} and @I{this is in italic.}. (Most commands start with @ - unlike TeX, this is just a convention.) However, in Lout language, everything can be fed into a command - Lout doesn't particularly care if you use @IncludeGraphic or use the powerful graphing commands to produce your own graphs, it all looks like a single unit to the language - and commands can take parameters from both sides of the command word. Thus, Finnish @Language{Tässä on lause} tells the interpreter to consider right side phrase to be written in language described at left side. While this sounds like a bastard child of Forth and TeX, in practice it seems far more interesting than operator overloading... Also, commands can take named parameters, as in: @Box linewidth {2p} {This is in a box.}
Lout has good library of tools. Basic formatting, including hyphenation in multiple languages (wow, Finnish too!), works very well. Lout also has capabilities to build tables and place figures, much like LaTeX. Bibliography database support is there, including conversion script from the de-facto standard BibTeX format. There are some very noteworthy features such as automatic "prettyprinting" formatting of some programming languages, extremely powerful construction of different kinds of graphs and diagrams (you can also use lout just to spew out these graphs in .eps format!)
Lout is regrettably not as famous as LaTeX or troff, so there's not as much material available about it, nor is there that much code available as add-on packages. There are still things that LaTeX does better - such as math formatting, even when Lout admittedly isn't far away, and LaTeX may also be more efficient at times.