Computer I.D.  Telephones have what's called "Caller I.D." You see the
number and sometimes even the name where the called originated. Is
there such a thing and how much does it cost to get it to see "who---
which PC" is or has visited my web site?

There is such a thing, known as an IP address.  An IP address, short
for "Internet Protocol," is a unique number consisting of 4 parts not
exceeding 256 separated by dots (e.g. 128.195.201.10) that are
assigned to each computer by their Internet Service Provider as it is
connected to the Internet.  There are two types of IP addresses:
static and dynamic.  A computer given a static address will retain
that same address everytime it is online, while a computer with a
dynamic address will change its address every time it gets online.
Hypertext Transfer Protocol, otherwise known as HTTP, is the primary
method of communication between computers and servers on the Internet.
 Everytime a computer requests a data file from a server, the server
will know the IP address of the requesting computer, along with some
other information such as: which web browser it is using, which
operating system it is running, which language is the user's preferred
language, which page referred the computer to your site, and many
other tidbits.
If you have access to the log file of your HTTP server, you should be
able to see who has visited your site and which files they requested. 
Once you know an IP address, there are free reverse IP lookup services
that can tell you whence they are from.  Unfortunately, most would
only be able to tell you the ISP that the user is using and their
geographic locations, but many can tell more.
This site: http://remote.12dt.com/rns/ resolves an IP to words, and
this site: http://www.arin.net/whois/ can give you more information
about who registered the IP addresses and where they are from.  Keep
in mind that if a user's IP resolves to AOL's, whose registered
address is from Richmond, Virginia, that does not mean the user is
from Richmond as well.  However, if a user's address resolves to
Harvard's network, chances are that the person is associated with
Harvard and is located in or about Massachusetts.
If you do not have access to the log file of your server, you can do
this with a PHP or Perl script (freely available) that records
information about your visitor everytime they request the page.

---

Clarification of Answer by secret901-ga on 27 Jul 2002 15:05 PDT

Search on Google for "Reverse IP lookup" for more services, most of
them are free.  It is unwise to use a paid service since they can't
tell you more than what the free services will tell you.
://www.google.com/search?q="Reverse+IP+lookup"

---

Clarification of Answer by secret901-ga on 27 Jul 2002 15:19 PDT

Like with a telephone "caller ID," it is entirely possible to screen
out unwanted people or give them different messages depending on what
you know about the user.  For example, if you offer software for
Windows and the client computer is using a Mac, you can then give a
message that notifies them of that.  Or if you don't like AOL user for
some reason, you can just deny access from all people using AOL and
tell them that they're not welcome :-).  A more pertinent example for
your case is that if your server detects that the user's preferred
language is Spanish, you can show them a Spanish web page instead of
an English one.  In fact, if you visit Google's home page at
://www.google.com, the language that shows up is the preferred
language that the user have set up in their browser.

secret901-ga  You have done an excellent job. What amazes about your
Google Answers Researchers is that you are so THOROUGH!!! You got me
web sites, special codes, how-to's, etc. I am so glad you answered me.
Now I know where to go and what to do!!! Great!

God bless you to know and do His will!!
John P. in the mountains of Mexico