Being as I am an OLDER CITIZEN of the republic, I really like free Handy Calculator -- http://www.macupdate.com/app/mac/38524/handy-calculator -- due to its simple, uncluttered but sleek black design, large numbers, and its ability to show accumulative calculations in its display string. I find Handy Calculator much nicer than Apple's default calculator. What an eyesore that one is! Where did the unified theme go, Apple? :(
Mind you, I only use a calculator occasionally, and for very simple mathematical computations. Thus, Handy Calculator meets my needs just fine.