Acqualia's Soulver, a very cool "calculator scratchpad" (which we've covered before), has come to the iPhone [iTunes link]. Both versions allow you to enter expressions in regular mathematical form, or a more "natural language" form. It keeps a running total in the sidebar, and you can reference (and edit) previous lines. Of course, it can run basic functions like sin, cos, and tan, but you can also use "English" functions such as "$29.99 - 35%" to get sale prices, markups, calculate tax, etc.. As a handy touch, Soulver (both iPhone and desktop versions) also allows you to save your work in files for later reference/editing.
Soulver has always been a great alternative to standard calculator programs, and the iPhone version is -- in many cases -- a full replacement for the built-in calculator. Check out the iPhone version ($2.99US) on the App Store, and the desktop version ($19.95US) at the Acqualia site.