Let me suggest one candidate for underhyped software – that is software that is quite good but just manages to stay under the radar except for the people doing work with it – the ones in the know. The JavaScript framework jQuery is just such underhyped software.

jQuery Selector lab demo from the book jQuery in Action

jQuery is like the APL of HTML -it is smart about passing arrays of DOM selector tree results from function call to function call like so:
which finds all <div>s with class=”highlight”, causes them to fadeout from the Web page, and then adds the class=removed to those <div>s in the DOM element tree.
This APL-like form allows jQuery to produce very concise JavaScripting of both AJAX connections and UI widgets. It also is fairly well behaved with other JavaScript libraries. Again – check this software out if you are involved in Web and AJAX development.