I have a server running on port 80, but I do not know what it is or where it came from. When I run
sudo lsof -i :80 | grep LISTEN
httpd 80 root 5u IPv6 0x91f5a9de62859cfd 0t0 TCP *:http (LISTEN)
httpd 694 _www 5u IPv6 0x91f5a9de62859cfd 0t0 TCP *:http (LISTEN)
Like Gordon suggested, that's the built-in version of the Apache web server.
You can stop it with
sudo apachectl stop
btw, the configuration for this webserver can be found in the /etc/apache2/httpd.conf directory.