How do I find out what my external IP address is?

Question

My computers are sitting behind a router/firewall. How do I programmatically find out what my external IP address is. I can use http://www.whatsmyip.org/ for ad-hoc queries,

Answer 1

Another way is if you have access to a cloud email (yahoo, google, hotmail), send yourself an email. Then view the headers and you should see your IP address in there.

I would look up the exact area but the headers may vary from each implmentation, Look for the received-by and follow that until you get to something that looks like sent-by

EDIT: This answers the how to find IP address, not the via PROGRAMMATIC approach