Telephone Directories

The Ultimate White Pages and Yellow Pages lets you search several directories simultaneously.

SuperPages.com - from Verizon (formerly Big Yellow from Bell Atlantic).

Switchboard.com

Toll Free

U.S. Toll Free Numbers - from Internet 800 Directory

Government Numbers
Government Information Xchange

International Telephone Directories

Canadian Telephone Directory

Teldir.com, formerly called Buttle and Tuttle, Ltd.

Superpages Global Directories

Area Codes

Area Decoder

Listed by number

Listed by state/country

Reverse Look up

Reverse, or crisscross, telephone number lookup.
For reverse look up by address, go to the bottom left side of the screen and click on "Reverse Address Directory".

Email and Website addresses
There are not yet really comprehensive email directories available on the Internet. You may have some success with these.

People.Yahoo

WhoWhere.Lycos
To find out who a Website is registered to, try:

Register.com

InterNic Registry Information

Zip Codes and Postal Codes

United States Postal Service Zip Code Directory

Canada Post Corporation

International Postal Codes Scroll down the page to listing for the various countries.

Professional and Special Directories

Martindale Hubbell Law Directory

West Law Directory

American Medical Association search by state

Massachusetts Board of Registration in Medicine - Online Physician Profiles

Museums of the World

Art Museums Network

Additional Internet links/pathfinders for this subject:

Librarian's Index to the Internet - click on "Organizations"

Internet Public Library - Associations

About.com - In the search box, type "Associations" or "Organizations"