If it were possible on a site (and I think you'll find that 99.99999% of sites won't allow it) then just entering the address (e.g.
http://www.apple.com/ ) would give you what you were looking for.
As I say, though, you're more likely to be out of luck because of the way web servers deal with requests. Most modern servers will attempt to serve an index.* or a default.* page if no page is specified in the address bar. Only if their default files are not found will they give you a listing of available files.
Also worth noing is the fact that the list of files you get back may well not be a complete list of the files that exist, depending on how the server's owner has set it up.