See http://wiki.luatex.org/index.php/TeX_without_TeX for an introduction.
I have (shameless plug) created a database publishing software using this technique (https://github.com/speedata/publisher/). Once in a while I have to use LaTeX and it feels a bit old school to do the macro programming.
My next project is to rewrite the TeX algorithms in Go - see https://github.com/speedata/boxesandglue. Already usable but not TeX like in any way (this is just a library, not a frontend software like TeX)