Basically this topic has been covered.
But I feel like adding my two cents in.
Getting some ones IP is fairly easy to do VIA Yahoo or AIM.
For Yahoo there are a few methods.
1.) Using a program (IP Sniffer Pro) plug plug....
2.) Doing it manually.
To do it manually as some one else already posted you need to run DOS.
And from the DOS Prompt type Netstat -N.
When the active connections are listed you will be looking for a TCP Port
like 5101 or 5100.
And that is the IP address of the person you are in chat/super webcam/file transfer with.
For Aim, the process is also quite simple.
Open up an Instant Message, and click on the picture tab.(Direct Connection)
After they have accepted load up good old DOS and again type NETSAT -a.
I don't know the actual TCP Port for AIM its been a while.
It might be 5190 but im not sure.
Well thats my two cents.
-UnSaKreD
http://www.unsakred.net