CompCalc is packed full of some neat and unique features. As a engineer, my inital desire was for a calculator in which I was able to work in standard engineering notations, like "k" for "kilo", "M" for "Mega", etc. As a programer though, these these units are sometimes "Base 1024", as opposed to "Base 1000" - for example, "Kilo" really means "1024" on a computer, not "1000". They now call this an "SI" convention, but few people use the new "SI" terminology.
Another great feature of CompCalc has always been the "tape-roll" type of display, where your whole session is displayed for you to go back and look at, like an old-fashioned paper-tape adding machine, (or automatically re-enter data from). The inherent way that the iPhone handles scrolling text fields and keypads made the iPhone version all that much more easier and intuitive than the older Blackberry, Windows, and Windows Mobile versions.