MiKTeX is an up-to-date TeX implementation for the Windows operating system. TeX is a typesetting system written by Donald E. Knuth, who says that it is "intended for the creation of beautiful books - and especially for books that contain a lot of mathematics".
Design Science MathType™ for Windows and Macintosh is a powerful interactive tool that will revolutionize the way you create print and web-based documents that contain math. MathType works with any word processor, presentation program, page layout program, HTML-authoring tool, plus other types of software, to create equations for research papers, class materials, web pages, slide presentations, journal articles and books.
Download MathType for Windows Font Installers.
Several people mentioned that it'd be nice to have some way to convert RTF (Rich Text Format) files into HTML. rtf2html will convert Microsoft Word documents using a specific style sheet into HTML.