InnoEx is a free tool which displays the files within Inno Setup installers and allows them to be viewed or extracted individually.

Basic usage is simple. Drag and drop your "setup.exe" file onto the program and it displays everything the installer contains (or an error message if this isn't an Inno Setup file).

You could use this as a simple viewer. If there's a changelog.txt file, for instance, you can double-click to see the latest changes without having to install the program first.

It's also possible to select one or more files and extract them to a folder. This may enable running a program without using the full installer, although there are no guarantees.

Perhaps the most interesting, if complex option is to view the install_script.iss file. This tells the installer exactly what to do, and allows you to see which files and Registry keys an installer will create.