The empheq package is part of the mh bundle. The package provides a visual markup extension to amsmath. The user-friendly interface allows the user to put a set of equations inside a box thus enhancing the \boxed feature of amsmath. As a side effect it's also possible to add material on both sides of the equations thus providing (and surpassing) the functionality of the cases package. Users of ntheorem will probably want to have a look at it as well, since the problem with end-of-theorem marks in gather and other environments can be circumvented using empheq.