Draw and make calculations on chemical structures
Chemtool is a GTK+ based 2D chemical structure editor for X11. It supports many bond styles, most forms of text needed for chemical typesetting and splines/arcs/curved arrows.
Drawings can be exported to MOL and PDB format, SVG or XFig format for further annotation, as a PiCTeX drawing, as a bitmap or as Postscript files (several of these through XFig's companion program transfig).
The package also contains a helper program, cht, to calculate sum formula and (exact) molecular weight from a chemtool drawing file. Cht can either be called directly by Chemtool or on the console.