The quickest and most reliable way to find someone's email address is usually to phone the person up and ask! Other than that, there are several online resources that can help track down friends or relatives online:

If the person you're looking for is a member, you can go to the Member Directory and search using their name or town/city, etc. If you find their screen name, that's all you need in order to email them.

If the person you're looking for is not a member, things are a little trickier. Because of the speed at which the Internet is growing, there's no definitive 'phone book' of email addresses, however, there are several Web sites that come close. Try some or all of the following:

-- WhoWhere: http://www.whowhere.com
-- World Email Directory: http://www.worldemail.com
-- Internet Address Finder: http://www.iaf.net