Thorpe's solution is the most obvious.
For security's sake, i.e. someone gets a hold of a person's e-mail account and decides they want their passwords, you could instead reset the password and have the new password mailed out.
To use thorpe's solution, you would need to have a DB of unencrypted passwords, or passwords using a reversible cryption. How do you store passwords in the DB? Just plain text? MD5/SHA1? If you encrypt your data, chances are you'll be using my suggestion. If not, try thorpe's.