Ghemical!

Avogadro is a free, fantastically easy to use molecule editor.

Marvin is a great alternative to the subscription based ChemDraw by CambridgeSoft. It is a collection of tools for drawing, displaying and characterizing chemical structures, substructures and reactions.It will also convert structures you create to various molecular visualiztion formats, such as .pdb and .mol, compatible with programs such as PyMOL and SwissPDB Viewer.

XDrawChem is a free software program for drawing chemical structural formulas, available for Unix and Mac OS. It is distributed under the GNU GPL. In Microsoft Windows this program is called WinDrawChem.Major features Fixed length and fixed angle drawing Automatic alignment of figures Detection of structures, text, and arrows, and their automatic placement Can automatically draw rings and other structures - has all standard amino acids and nucleic acids in a built-in library Retrieval of structures from a network database based on CAS number, formula, or name Retrieval of information on a molecule based on a drawing Symbols such as partial charge and radicals Reading MDL Molfiles, CML (Chemical Markup Language), ChemDraw binary format, ChemDraw XML text format Writing MDL Molfiles, CML, ChemDraw XML text format Integration with OpenBabel, allowing XDrawChem to read and write over 20 different chemical file formats. Image export in Portable Network Graphics (PNG), Windows bitmap, Encapsulated PostScript (EPS), and Scalable Vector Graphics (SVG) 3D structure generation with the help of the external program BUILD3D Simple spectra predictions, including 13C-NMR, 1H-NMR (based on additive rules and functional group lookup methods), and IR Simple property estimation, including pKa, octanol-water partition coefficient, and gas-phase enthalpy change. »

Jmol is a free, open source molecule viewer for students, educators, and researchers in chemistry and biochemistry. It is cross-platform, running on Windows, Mac OS X, and Linux/Unix systems.

Calistry is an online Chemistry Calculator. Till date Calistry contains 70+ calculators in the field of Chemistry. Everything in the website is entirely free to use. Other than calculators, Calistry also provides online molecular viewers that visualizes coordinate file of leading molecular dynamic pakages.

What's your favorite molecule? With ChemJuice, chemical structure drawing is now at your fingertips.