..and in the administration page, under System Admin, Preferences, General Settings: Set the "Debug Mode" to "Smarty Templates Debug" and you'll get a pop-up when you browse a module.
If you are using Fire-Fox, this will trigger it's pop-up blocker, just allow pop-ups for the site.
You'll see a list of all the smarty vars available for the page you are browsing.

I do agree that more of the raw vars should be available. I sometimes add vars to modules but this makes extra work at upgrade time as I have to re-do the hacks.

Does anyone know if there is a guide for module writers from the smarty vars point-of-view? Something like:

If the module has categories:
Include vars for the id and name in the form $modulename_catid and $modulename_catname.

If the module has items:
...