One of the assignments for "Visual Programming" course in the university was to write a calculator. This one is not yet another boring desktop calculator: you perform calculations by typing mathematical expressions instead of pushing buttons. Of course, you can push the buttons too, if you want it. ;)
(I compiled it and took this screenshot in Linux, using WINE, that's why it looks slightly strange.)
The first archive contains the sources (Borland Delphi), the second contains the compiled executable. This program is GPLed (see gpl.rtf in the first archive or use the "?" button in the program for details).