the FSF version of gnat is robust, mature, and reasonably up to date. It's licensed LGPL so you can use it pretty much like anything else. Adacore also puts ou

the FSF version of gnat is robust, mature, and reasonably up to date. It's licensed LGPL so you can use it pretty much like anything else. Adacore also puts out a slightly more state of the art version of gnat but it's GPL so in theory everything you make with it needs to be gpl as well (though of course that's fairly irrelevent for many classes of software)