LaTeX
Scientists the world over have a long and rich history of writing books, papers, and journal articles to share ideas and results with colleagues. The ability to share these writings was greatly increased with the advent of (western) moveable type and mechanical printing, both developed by Johannes Gutenberg. The availability of "cheap" printed material increased both literacy and general education levels among the masses.
Skip forward a few years to the age of typewriters. These machines produced nice, legible characters with regular spacing. Typists could even write faster with these machines than by hand, a clear improvement in the writing process.
Skip forward a few more years to the "intelligent" word processors and microcomputers. These machines were initially designed to replicate typewriters, but were able to include more special characters, fonts, font sizes, and eventually, grammar and spelling checks. While this was practically a revolution in the way that most people composed documents, the major downfall of these computers, and the software they used, was their inability to do really complex characters and typesetting, which was still done by hand or by highly specialized machines.
Enter TeX and other similar typesetting software packages (troff, groff). Instead of using the, shall I say, "user-friendly" WYSIWYG format, users instead would code their documents by using a markup language. The benefits included automatic indexing, page numbers, and very pretty scientific and mathematical typesetting. (To be fair, WYSIWYG programs such as Lyx, TeXmacs, the large array of desktop publishing suites, and even Microsoft Word, are now offering competitive features.)
All the documents I write these days are done using LaTeX/TeX Live in the TeXShop editor for Macs. There is a non-trivial learning curve for TeX and it's various distributions (LaTeX, teTeX, MikTeX, TeX Live, MacTeX, XeTeX, XEmTeX, proTeXt, etc), but for anyone who needs to write a lot of math, or for anyone who really appreciates elegant output, TeX is the way to go.