PeekYou is a people search that currently claims to have 143,839,270 profiles online. You can search by name, by work or by school attended.
While it can't really make up its mind if it's a social interaction site or a business profile search, PeekYou does have a number of useful features. One is "Possible Relatives," with a list of those sharing the same last name and present or past location.
PeekYou has a lot in common with other people-search sites with a business bent, including LinkedIn, ZoomInfo, and Spoke. Similar sites that are more geared towards social interaction are of course MySpace and Facebook.