For serious searchers willing to pony up a fee to locate someone, or to find out just how much information is out there about themselves.

1-800-U.S.Search (an address will cost you $79; a background search goes for twice that)
www.1800ussearch.com/

A1-Trace USA (lots of info, including SSNs and criminal records, for $79)
a1trace.com/

Discreet Data Research (someone trying to skip town and hide? $300 finds them)
www.discreetdata.com/

Discreet Data Systems (find unlisted phone numbers for $65; for $650, you can perform a complete asset search, including bank, real estate, and employment records)
www.discreetdatasystems.com/

Lexis-Nexis (the gold standard of record searches)
www.lexis-nexis.com/

Public-Records (net SSNs, addresses, and driver's license/VIN traces for the price of two six-packs)
www.public-records.net/

Search Systems (links to U.S. public record databases of all kinds, much of which is free)
www.searchsystems.net/