Yale University
Library/Internet Search Engines
The best advice for finding out someone's email address is to ask them directly! There are, however, alternative (and more cumbersome) ways of going about it.
Universities or Corporations or other Organizations
There is a perfectly good chance that the organization that the person works for maintains an online directory of email addresses and/or phone numbers. Use Yahoo! or Alta Vista to get to the homepage of that organization. At Universities, these directories are usually called CSO phonebooks.
Finding an Email address - http://sunsite.oit.unc.edu/~masha
This document is a good introduction to the various internet tools for detecting people's email addresses. It discusses WHOIS, FINGER, and other directory services.
Yahoo! People Search - http://www.yahoo.com
A strange little feature in the advanced search mode of Yahoo! is the "E-Mail Addresses" button. Click this button and type a name in the box, and a list of likely addresses will appear.
Netscape People Search - http://home.netscape.com/home/internet- white-pages.html
A collection of links to a number of online directory services. Not all of these include email addresses, but it is another place to begin hunting.