<p>Are there any free tools available to view the contents of the solution user options file (the .suo file that accompanies solution files)?</p>

<p>I know it's basically formatted as a file system within the file, but I'd like to be able to view the contents so that I can figure out which aspects of my solution and customizations are causing it grow very large over time.</p>

I don't know any tool, but you can try to access user settings via
IVsPersistSolutionOpts
interface