ConTeXt is a very powerful, elegant macro package for TeX, written by HansHagen.  It does the same things as LaTeX (and more).

(This isn't the ConTextProgrammersEditor created by EdenKirin.)

Major advantage of C''''''onTeXt over LaTeX: C''''''onTeXt is (approximately!) 100% compatible with Plain TeX: you can intersperse C''''''onTeXt and Plain TeX as much as you like.  L''''''aTeX isn't compatible with Plain TeX (amazingly).  So it's much easier to extend C''''''onTeXt yourself than it is to extend L''''''aTeX.

See http://www.pragma-ade.nl/ and http://home.salamander.com/~wmcclain/context-help.html.

-- JasonGrossman

Some nice features:
* Can completely replace Notepad.
* Maps file extensions to external execution scripts.
* Massive collection of SyntaxColoring/highlighters