You may want to take a look at the METAS VNA Tools by the Swiss national metrological institute:
https://www.metas.ch/metas/en/home/fabe/hochfrequenz/vna-tools.html.
You have to register if you want the full version, but there is also a reduced one (VNA Data Explorer) free to download, which only contains the data plotter. The full version adds a lot of functions like offline calibration and S-parameter uncertainty evaluation according to the EURAMET calibration guide.
As a data plotter the VNA Tools are very convenient and hard to beat in terms of speed and versatility. They are ideal if you quickly want to look at different Touchstone files and make comparisons, etc. You can pot linear/logarithmic magnitude, phase, convert to Z-parameters, plot in one or several diagrams, take differences and quotients, do time domain analysis, calculate passivity and switch terms, etc. The user interface is a bit unusual, but actually very powerful, especially with the integrated file manager. The only lacking feature is a Smith chart plotting facility. You can however plot \$S_{nn}\$-data as a polar plot with rectilinear coordinates, but not with the curved Smith coordinate lines. For me this is not a real shortcoming as long as I only want to look at S-parameter data.
As was already mentioned, the most flexible option would be a math package like MATLAB, Octave, Python (scikit-rf) with good plotting capabilities. Then you can do arbitrary math on the data, the plots are infinitely customizable, and can be output in publication quality in any format you want. Takes longer to set up, though.
A third option is to use a simulation package with S-parameter simulation capability. I often use QucsStudio, which is free and very powerful:
http://qucsstudio.de/de/start/. Besides plotting this allows to add circuit elements to your S-parameter data. Also takes longer to set up and is not too convenient for browsing files real quick.