Since today is the first day of school for alot of students here in Las Vegas, I thought it would be good timing to point you to the Magic Number Machine.
From the developer's site:
A free, full-featured, graphically laid out, high-precision, scientific calculator for Mac OS X 10.2 and greater. Full source-code is included with the distribution.
Just looking at the sample equation on the homepage reminds me why I'm glad I'm not in school anymore.