FlexiSheet is a multi-dimensional spreadsheet in the spirit of Lotus Improv.
FlexiSheet runs on MacOS X.
Nice and simple.
Note there is a GnuStep port (i.e. for other operating systems) at http://cvs.savannah.gnu.org/viewvc/gap/user-apps/FlexiSheet/?root=gap And you can see the old website at http://web.archive.org/web/20050308133602/http://www.materialarts.com/FlexiSheet/index.html
this is a great project.