We were trying to solve an annoying bug in our mod_rewrite code today and couldn't quite figure out why our server was acting the way it was.  Fortunately we discovered Web-Sniffer.net.  Web-Sniffer.net allows you to view the HTTP request and response headers for any URL you like.  You can even choose between HTTP/1.1 and HTTP/1.0, with and without the Host header.

View HTTP Headers

The best part is that this is much easier than messing about with Telnet.exe!