Ask them for it. Otherwise it may be illegal to get it any other way.

Are you talking about individuals or sites? Kind of like the nslookup command?

Why?

If you are looking for a websites IP you can just ping it and it will reveal the IP.
If you are looking for a friends IP, you must do like user 'califdadof3 said, ask them for it.

To Ping, open Command Prompt, under, Start -> All Programs -> Accessories - > Command Prompt
In the black windows, type ping and the address, ex (ping www.google.com).

Remember, if you are trying something illegal, the authorities can track you REAL easily.