I assume you've tried Googling the address? Give both Google and Yahoo a shot. If the person's used the address anywhere - like in a profile on a public page/site or on a message board (such as this one) - and it's been indexed, that might help ya get a lil more info on it.

A lot of free, web-based email services also have other services tied to them, as well. Like if someone uses their hotmail address for their MSN community profile, or their Yahoo! address is the same they have a Yahoo! people profile for.

Those are some pretty basic, totally legal, and none-too-intrusive ways.