*sigh*
Many people keep mailing me questions or suggested hacks to get
visitor information, particularly email addresses.   It seems they
won't take "NO" for an answer.

The bottom line is that whatever information is available to _you_
is _equally_ available to every spammer on the net.   Therefore when
a browser bug _does_ permit personal data to be collected, it gets
reported and fixed very quickly (one short-lived Netscape 2.0.x
release reportedly had such a bug in its Javascript engine).

You can get some limited information from the environment variables
passed to you by the browser.   Relatively few of these are guaranteed
to be available, and some may be misleading.   For particular types
of information, see below.   For full details, see NCSA's reference pages.