Emacs comes with a couple of built in spreadsheets: ses-mode supports emacs lisp formula in cells, org-mode tables allow simple formulas in table cells. Furthermore, there are half a dozen or so other packages that users have written that are older or are more specialized in some way. See http://www.emacswiki.org/emacs/SpreadSheet.

All of these spreadsheets are, naturally, written in emacs lisp.