sudo lsof -n -i :8080 -s TCP:LISTEN
The first two columns will be the name and PID of the process(es) listening on port 8080.
Use the PID to find it in Activity Monitor, Get Info, and notice which files it has open. It'll have a bunch of libraries open, but high on the list will be the executable file.
Alternatively, from the name, see what service it corresponds to with
sudo launchctl list | grep theprocessname
For example, if the name were
httpd, the above command would tell you the service name is
org.apache.httpd. If you don't get any hits (because it's not running as a daemon), try it without
Ask launchctl for more info about that service. Continuing to use Apache as the example:
sudo launchctl list org.apache.httpd
would tell you
"LimitLoadToSessionType" = "System";
"Label" = "org.apache.httpd";
"TimeOut" = 30;
"OnDemand" = false;
"LastExitStatus" = 0;
"PID" = 39660;
"Program" = "/usr/sbin/httpd";
"ProgramArguments" = (
Notice the "Program" line, which gives you the path to the executable. Most of that information comes from /System/Library/LaunchAgents/org.apache.httpd.plist, which is the file that tells Launch Services when to run the program and how.
Your web server, listening on port 8080, is not from Apple, so it won't be in /System/Library. Look in /Library/LaunchDaemons, or maybe even in ~/Library/LaunchAgents (if it runs only when you're logged in). (Listening on port 8080 does not require root privilege, so it could conceivably be running as you, although that has interesting security implications.)
Another place to look is in System Preferences→Users & Groups→Login Items. Since we don't know how you set it up to run automatically, it might be that you didn't do it by putting a file in any of the ...Library/Launch* directories.