Here's an extract from Jarmo Lahtinen's post on Beta Labs:
"Have you noticed that even in the latest S60 devices the calculator application is very simple and limited in features? And have you noticed that in Series 40 devices there is a much better calculator application? The calculator in Series 40 devices is a Java application, and we started to think here about replacing the current S60 native calculator with this one. So, I took the source codes of the original Series 40 application and updated it a little bit. The end result is here, Java calculator application for S60 devices!"
The enhanced calculator will work on all QVGA S60 phones.
Three modes: standard, scientific and loan.
The standard calculator can be used without the need to select buttons on screen.
The new calculator is written in Java (rather than native C++); this is because its Series 40 ancestry. On some S60 phones (S60 3.0 especially), there may be a slight lag on start up. However more recent phones should have no such problems (e.g. the N78 was less than a second).
As a reminder here the standard S60 calculator (note how you need to move around with the D-pad to select the operators):
If you're looking for a simple calculator application (and don't think you'll need scientific or advanced modes) then check out Calcium as an alternative.