Security Growler 2.2

Security Growler is a menu bar app that will notify you via Notification Center or Growl if any SSH, FTP, VNC, AFP, or Sudo authentication events occur.

It's very useful if you're paranoid about people trying to hack into your computer or if you simply like having information about people using your computer's resources. It's easily extensible in python, you can add modules that watch logfiles or processes and do whatever you want. Use Prowl to send the notifications to your iPhone/iPad when you're away.