Hi I just want to know how you can tell what OS/Server software an
online webserver is running on. Is there a website that I can put the
domain-name/IP address into and it will tell me? Or a command line
tool? I can't tell from the page extensions because they are simply
.html (the website obviously uses a re-writing engine). Your very
prompt answer is appreciated!

Just try to view a non-existing page.
Like www.server.com/no_such_page.html

The result will be an error, and the Server name will be provided at
the bottom of the page.

Hi shadow_developer-ga. No unfortunately 404 are already being handled
- i.e. I get a friendly 404 which does not have any mention of server.

1. Visit http://www.netcraft.com
2. Enter your domain in the top-left box and press search
3. Look at the 'Hosting History' section on the results page
4. The latest entry will be the server you are running

I hope that works for you.

Thanks

PS. It's a shame normal GA users can't be paid for answers :-(

Try using ethereal or any other sniffer to monitor the HTTP messages
you receive from the website you want to check. The HTTP response
messages from website  would have a response-header field by name
'Server'. This field contains the tokens to identify HTTP server
software.

E.g. on monitoring HTTP respones from www.ebay.com, it reveals 

Server: Microsoft-IIS/5.0

See if it works for you.

This telnet command:
telnet  www.google.co.uk  80

Followed by the input
GET / HTTP/1.0   [and 2 newlines]

Produced the following, which you can see contains the name of a server stated
to be the one in use.   Websites are not obliged to be truthful in this.

HTTP/1.0 302 Found
Location: ://www.google.co.uk/
Set-Cookie: ...details...cut...
Content-Type: text/html
Server: GWS/2.1
Content-Length: 154
Date: Fri, 23 Sep 2005 21:48:01 GMT
Connection: Keep-Alive