A new version of gcalctool has been released
gcalctool is the default GNOME desktop calculator.Download
It has Basic, Financial and Scientific modes. Internally it uses multiple precision arithmetic to produce results to a high degree of accuracy.
This release is because I haven't done one since the GNOME 2.6 FINAL call for tarballs.
[Note that gcalctool now requires the Gtk+ that comes with GNOME 2.6 or later in order to build].