If you have someones IP address, you can google it, and use an IP tracer/tracker to find the location of their Internet Service Provider. If you're good at detective work, you might be able to figure out their precise address as well. You can also look for peoples houses, enter their username and find ALL of their internet accounts (if they're one of those people that use the same username for every site they visit). This can also work with youTube username if you want to uncover very old comments that they posted. If you have someones telephone number, you can google it...and see the result? *cough* REVERSE SEARCH *cough*
Reverse-searching a phone nomber only works in the USA though, so I'm safe