Most chemical structure drawing packages allow you to calculate molecular weight, also known as molecular mass, for any structure you draw. In no particular order:
Of course, if you have the mol file or an equivalent you should be able to open any structure with an appropriate viewer, including something like good-old Chime (from MDL) or the open source JMol or any of a dozen other viewers. If you want to create beautiful 3D images then Crystal coupled with Rasmol might be the way to go.
For more chemistry software, including a plethora of molecular weight and other calculators check out the Umea website