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 ConTeXt over LaTeX: ConTeXt is (approximately!) 100% compatible with Plain TeX: you can intersperse ConTeXt and Plain TeX as much as you like. LaTeX isn't compatible with Plain TeX (amazingly). So it's much easier to extend ConTeXt yourself than it is to extend LaTeX.
See http://www.pragma-ade.nl/ and http://home.salamander.com/~wmcclain/context-help.html.
Some nice features: