LaTeX is a high-quality typesetting system, with features designed for the production of technical and scientific documentation. LaTeX is the de facto standard for the communication and publication of scientific documents.
More information: http://www.latex-project.org/ and http://www.macdevcenter.com/lpt/a/4589 (02/03/2004) which covers Latex and the "Integrated LaTeX Environments" available for the Mac.
TeXShop (open source cocoa project) is a very good front end for TeX and LaTeX for OS X: http://darkwing.uoregon.edu/~koch/texshop/texshop.html .
ITexMac (open source cocoa project) is a very good front end for TeX and LaTeX for OS X: http://itexmac.sourceforge.net/ .
TextMate also has pretty nice TeX support, using the PDFBrowserPlugin it can preview documents without leaving the application: http://macromates.com/movies/html_pdf_output.mov .