IPSentinelStatus is a menu bar item which displays and copies your machines IP addresses and name. Can optionally send email when The IP address changes, and display the external IP address in the menu itself.
Using the the system preference pane you can configure the program to deliver email notifications of changes of IP address.
Put the application into your login items if you want it to launch automatically whenever you log in. Go to System Preferences "Accounts" and then click the "Login Items" pane. Then simply drag IPSentinelStatus into the list.