Growl notifications when a user logs in to Minecraft server?

The minecraft server writes a log file into the directory minecraft_server.jar is installed in. From there you can use something like

tail -f server.log | 
  while read m_date m_time m_type m_user m_ip m_action m_rest; do
    if [[ $m_action == logged ]]; then
      growlnotify -n Minecraft -m "$m_user logged in at $m_date $m_time" Minecraft;
    fi
  done

Starting this automatically together with your Minecraft server is possible but depends on the way you do start it.

To install this (without going too deep into Unix territory, ask an expert if it fails):

• open Terminal.app
• change to the directory where minecraft_server.jar is installed by typing cd followed by drag&dropping the folder with the server into the terminal window followed by return
• nano mc2growl
• write #!/bin/bash and press return (twice)
• paste the code from above
• press ^X and say Y at the prompt
• chmod +x mc2growl
• ./mc2growl &