Someone on either end of your connection is going to have to parse the data into an HTML page that blends into their site. The suggestions above are ideal if they can parse XML or comma delimited data.
Another way, if you are dealing with non-CGI people, is to get a template of their page from them and set up a cgi script that takes the parameters they want, then outputs the page formatted for their site. They could use a frameset for seamless integration. The disadvantages are much heavier load on your system and you would have to make changes if the templates changed (unless they left the templates somewhere and your script checked them periodically for updates).
Another option that might be beyond your skills, but very useful to learn and play with would be to set up a socket to listen for specific commands. You could then output pre-formatted HTML, XML or whatever they request and they would not have to do too much or any extra processing. I say VERY specific commands because you do not want to allow raw sql or server or perl into the socket, just a limited range of commands you make available for their needs.

I realize these suggestions are pretty vague, I just wanted to give you some ideas to think about.