Background: While studying Group Theory ( Open University M208 ) I had a lot of benefit from the Mathematica Add-on package AbstractAlgebra and later from the GAP software. I am currently self-studying Galois Theory ( using Ian Stewart's Galois Theory ).
Question: Is there a program that calculates the Field Extensions / Galois Group for a ( simple ) polynomial ?
I haven't checked it out myself, but have you looked at "Computing Galois Groups with Mathematica" in the Wolfram Library Archive? From the description, it may do what you want: