I have only tried the free ones, and frankly have had limited success. Hope you have better luck finding who you're looking for!

GENERAL SEARCHES

Welcome to Phonebook.com
Search for someone
AnyWho: Find People - Find E-mail - Toll-Free Directory Search
Yahoo! People Search
Welcome to WhoWhere?!
Phone & Address Search @ InfoSpace.com
AT&T Help/Search: Directories
Unlisted phone numbers needed, find someone's unpublished telephone number
Missing Friends Network Online Databases

FAMILY SEARCHES

LDS (Mormon) Genealogy NEW
"Kissing Cousins" Look for family
FamilySearch Internet Genealogy Service
Family History Information/Where Is the Nearest Family History Center?
Cyndi's List of Genealogy Sites on the Internet
Family Tree Maker's Genealogy Site
National Genealogical Society

ADOPTION SEARCHES

Adoption and Family links
Adoptees Resources Homepage
Adoption
Adoption Information Exchange
Adoption Network

OTHER SEARCHES

Find A Grave
Social Security Death Index