Qalculate! has all the features you expect from a calculator and has a fairly straight forward layout:
I like Qalculate! because in addition to being fairly simplistic in appearance it is also a powerful tool. Qalculate! supports creating your own functions and by default it comes with a slew of it's own:
As you may note from the menu Qalculate! also comes pre-programmed with a variety of scientific variables and various unit conversions (because the US is silly and still doesn't use the metric system!).
Beyond the functionality of Qalculate! it is also a well written tool because the calculations/tasks it performs are completed separated from the GUI front end. This means Qalculate! comes with more than one front end, currently it has both GTK and QT options (and I hope to see an EFL front end some day). If you have not yet tried Qalculate! out before I highly recommend it. You should be able to easily find and install it from your distribution's repositories (Bodhi users can also find it in the appcenter).