Here are some sites which we found useful:

Telephone Directories
Businesses www.yellowpages.com
Individuals www.whitepages.com

Reverse Search by Telephone or Address (US & Canada)
www.whitepages.com
www.phonenumber.com

Nearby (Neighbor) Searches by Street
www.skipease.com/neighbor_search.html

International Searches
www.whitepages.com/intl_directories

Searches By Profession - CPA Directory, Law Firm Locator, etc.
www.skipease.com

Unlisted Telephone Numbers & Cell Numbers - (Approx. $50.00/find)
www.unlisted-telephone-numbers.com

Public Library Locator
http://www.nces.ed.gov/globallocator/

Post Office Locator
http://www.switchboard.com/usps

Deceased Person Searches by Name, Address or SSN
http://www.ancestry.com/search/rectype/advanced.aspx

Internet Calculator
www.bankrate.com

Time Zone Issues
www.worldtimezone.com

Currency Conversion
www.xe.com/ucc/