ConTeXt is a macro package developed by Hans Hagen, originally to serve the needs of his (Dutch) firm Pragma. It was designed with the same general-purpose aims as LaTeX, but (being younger) reflects much more recent thinking about the structure of markup, etc. In particular, ConTeXt can customise its markup to an author's language (customising modules for Dutch, English and German are provided, at present).
ConTeXt is well integrated, in all of its structure, with the needs of hypertext markup, and in particular with the facilities offered by PDFTeX. The default installation employs a version of PDFTeX built with the e-TeX extensions, which allows further flexibility.
ConTeXt doesn't yet have quite such a large developer community as does LaTeX, but those developers who are active seem to have prodigious energy. Some support is available via a WIKI site.
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