Avogadro - Avogadro is a free, fantastically easy to use molecule editor.
ChemDraw - ChemDraw - The Gold Standard for Chemical Structure Drawing and Research Publications.
Gnome Chemistry Utils - The Gnome Chemistry Utils include six chemistry related programs:
ChemDoodle - Chemical drawing and publishing software for desktop, web and mobile. Affordable and used by thousands of scientists around the world. Free trial available for immediate download.
MolView - Open Source online service
MedChem Designer - Free chemical structure drawing program for Windows. Will also run in Linux using WINE.