As maxima is written in lisp it can be embedded in lisp programs like front-ends and there actually is a very good one that contains a maxima.
ECL can convert maxima to a C++ program that can be embedded in your own C++ projects. abcl can convert maxima to java. Even if ecl isn't exacly fast and abcl is even slower we should provide information on how to call maxima from inside a programm.