Achim Blumensath’s ANT project aims not to replicate TeX with a different implementation technique, but rather to provide a replacement for TeX which uses TeX-like typesetting algorithms in a very different programming environment. ANT remains under development, but it is now approaching the status of a usable typesetting system.
ANT’s markup language is immediately recognisable to the (La)TeX user, but the scheme of implementing design in ANT’s own implementation language (presently OCaml) comes as a pleasant surprise to the jaded FAQ writer. This architecture holds the promise of a system that avoids a set of serious problems with TeX’s user interface: those that derive from the design language being the same as the markup language.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ant