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You could try people.yahoo.com, but if they have a common name or you have very little information it will be tough to narrow down your search without a professional service.
I don't know any that will actually find people for free.
My wife was looking for someone she went to high school with. Sho looked on www.classmates.com which is where people can register for where they went to school. She found her friend.
At work detectives will often go to www.myspace.com it is a place to place a free web page on and tons and millions and millions of people have personal sites on there from young kids to old adults.
65,503,916 is the number that came off the myspace site for number of registered users.