So far I've been unsuccessful in finding an open source tool that will convert a large number (>100k) of SMILES strings to a chemical structure. Is this really only solved by commercial applications? I would not mind converting through a number of intermediate formats. Ideally I'd do it with R, but that is the least of my worries. It's also neither for personal nor academic use.
For example, the following code will give you a neat SVG file of the molecule benzene:
If you experience problems using it, you are welcome to ask more specifically.
Alternatively, you can use a web-query from the national cancer institute. It is easily accessible by the following code
Another open source solution, where you can directly export the structure into a molecular editor is Avogadro. (It uses Open Babel though.)
Depending on the actual problem, however, there might already be more advanced routines.
In addition to the other good answers, I'd recommend
Here are some
Here is one way to convert a SMILES to a structure in rdkit.
I'm surprised that you've had difficulty finding a toolkit - is it that the licence must be MIT or as permissive? I guess that you will be using this in software you are making, rather than a one-off data conversion?
For example, OpenBabel (C++), Chemistry Development Kit (Java), etc - in addition, the CDK can interface with R - would seem to suit your needs?