The software Open Babel lets you convert between lots of chemistry file formats, and do lots of other cool things like drawing pictures of compounds.
Here are some simple things I've learnt:
Drawing a picture of a molecule:
% ~jc17/software/openbabel/bin/obabel -isdf Structure2D_CID_9812710.sdf -xu -xd -xC -m -O ivermectin.svg
where Open Babel is installed in ~jc17/software/openbabel/bin/obabel
For some reason this makes you an output file called ivermectin1.svg
Converting an SDF format file to SMILES format:
% ~jc17/software/openbabel/bin/obabel tmp.sdf -O tmp.smi
To get the ids in the output file:
% obabel NNN.sdf -O NNN.smi --append IDNUMBER
To kekulize a SMILES string:
Run Open Babel version 3.0 with "-osmi -xk".
Note afterwards: Noel said normally molecules are drawn without showing all the hydrogens.