What HTTP request headers can I use?

Most HTTP request headers are passed to the CGI script as environment
variables.   Some are guaranteed by the CGI spec.   Others are server,
browser and/or application dependent.
To see what _your_ browser and server are telling each other, just use
a trivial little CGI script to print out the environment.   In Unix:
 #!/bin/sh
 echo "Content-type: text/plain"
 echo
 set
(Just call it "env.cgi" or something, and put it where your server
will execute it.   Then point your browser at
http://your.server/path/to/env.cgi ).
This enables you to see at-a-glance what useful server variables are set.
Note that dumping the environment like this within a more complex
script can be a useful debugging technique.
For details, see the CGI Environment Variables specification at
http://hoohoo.ncsa.uiuc.edu/cgi/env.html
(which also includes a version of the above script - somewhat more
nicely formatted - online).