Mac OS X: find the program running on a port

Sometimes, there's a program running on a port and you don't know what it is. How do you find out? I find this happens when I start a webserver up to test something locally and then I forget about it. So, if I wanted to find out what was running on port 80:
  sudo lsof -i :80 # checks port 80

        
          COMMAND   PID USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
          ruby    64888   wg    6u  IPv4 0x6ddd270      0t0  TCP *:gds_db (LISTEN)
  This command shows a list of open files. The -i option checks for internet addresses with the colon symbol representing a port (instead of an actual IP address). Note, I've only used this on Mac OS X. I'm not sure if there's an equivalent for *nix or if it's available in some package.

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