I use LaTeX for all my writing. LaTeX, originally written by Leslie Lamport, is a set of macros for TeX. TeX is Donald Knuth's powerful, versatile, programmable type-setting language "for the creation of beautiful books — and especially for books that contain a lot of mathematics" (Donald E. Knuth: The TeXbook, Addison Wesley 1984, p. v). TeX/LaTeX produces professional-quality typeset pages of any kind and is superb for scientific and technical works. The TeX showcase provides examples of what TeX, LaTeX, and related programs can do.
TeX/LaTeX provides excellent support for foreign languages and non-Latin alphabets. Multiple languages and alphabets can be used in a single document. TeX is strictly standardized and runs on most computer platforms. The web page of the TeX Users Group (TUG) contains information about freeware implementations of TeX for various platforms. Source files for TeX/LaTeX are plain text files. They can be prepared with any text editor and contain the text and embedded TeX/LaTeX commands. TeX/LaTeX source files can be exchanged easily between different computer platforms or be sent via e-mail. pdfTeX and pdfLaTeX generate PDF files straight from the TeX or LaTeX source files.
The LaTeX format is a set of higher-level commands for the production of documents from letters and papers to books. It automates several tasks, such as running heads, sectioning, tables of content, cross-referencing, equation numbering, citations, and floating of tables and figures. LaTeX is very extensible via the use of packages. Packages exist for many applications, from printing envelope labels with US postal bar codes to typesetting chemical structures to graphics.
LaTeX is not difficult to learn. With a good introductory book, e.g., H. Kopka and P. W. Daly: Guide to LaTeX, 4th ed. (Addison-Wesley 2004) or M. R. C. van Dongen: LaTeX and Friends (Springer, 2012), you can be productive after a weekend of study. I have been using TeX/LaTeX since 1993, and it has become an invaluable tool.
The Comprehensive TeX Archive Network (CTAN) is the central place for all things related to TeX/LaTeX. (La)TeX Navigator web site provides a wealth of information on TeX, LaTeX, Metafont and other related software and packages. Further help can be obtained from the TeX Frequently Asked Questions page.
The LaTeX package TikZ is an amazing tool for producing high-quality graphs and figures. The TikZ and PGF manual is very long and can be intimidating at first. To get started with TikZ, the short A very minimal introduction to Tikz (24 pages) by Jacques Crémer is very helpful. If you read French, I highly recommend TikZ pour l'impatient by Gérard Tisseau et Jacques Duma. Examples of figures and graphs made with TikZ, with the corresponding LaTeX code, can be found at TeXample.net.