MW Calc is an easy to use program to calculate the mass of a molecular formula. Features: allows lowercase or uppercase input; Simple, convenient interface; can look up any element by its symbol (convenient when you forget what element number Ta is); supports copying and pasting; accurate to 3+ decimal places, marks errors in your input for easy correction. Perfect for any Chemistry related profession. Now does isotope patterns, mass percents and AMUs, and amino acids! (See Update description).
Updated Description:

v7.0 Easier input: through element buttons or any element using grafiti. v6.0 Now supports Amino Acids! v5.0 Added Mass Spec isotope pattern calculator! v4.6 Added single precision calculations. Now you can lookup elements by their atomic number v4.0 Added Mass Percents (Mass %) for the elements in the formula. Added exact AMUs for Mass Spectrometry. Added Melting points, boiling points, and much more for each element. v3.0: Fixed two bugs. One quite major and one that was just an annoyance. Added support for Deuterium and Tritium. New: no hassle for registration! v2.5: Now supports spaces and it is easier to look-up elements by their symbol.