bosky thats a weird question man. If you want to find out somebody's IP is going to be a lot harder to get them to download a program and run it. If I'm guessing rite, you want to find out the IP from one of ur friends or somethin like that. If thats the case just do the following.
Download a free webserver, somethin small and easy to install. can't go wrong with Apache
Send ur friend an email or IM with a link to the site which would be http:// followed by ur ip (for example: http://127.0.01.)
When he clicks on the link jus read ur servers logfile an you got his IP.