I don't recommend classmates.com as they spam the crap out of you.
Myspace is hit or miss. I haven't found too many people on it.
Spock is a new service that is beta only and I haven't found any purpose for it, but I have the beta. It is supposed to be a people search. www.spock.com
I've heard a lot of talk about ZabaSearch. It searches publicly able government records as well as some commercial sources. A lot of people say that it violates their privacy because it's so easy to find people, so I guess it works pretty good.
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