I spent couple of hours searching for windows command line program which can provide external IP address but except payable services there is no reaible tool for this. So I wrote my own - you can download it here. Program is written in .net and can be also used as library or even compiled for mac or linux using mono. Although this is simple program it includes all options for reliable acquiring external IP address, because it not depends on single public site but many; use -? parameter for details.
You can use tool as you want, but if you like it then would be nice if you share this information and link to this page. Optional comments will encourage me for improvements also :-)