Please! Don't do that!
Implementing a login system on a web application is easy. Implementing a secure login system on a web application is a totally different matter. There are many aspects and details to consider, and Drupal does a fine job of dealing with just about all of those aspects out of the box.
Adding the feature you ask for would greatly reduce the security of your application. If you can decrypt the passwords, you have just stripped your app of a very important security mesure.
Other answers explain workarounds, BetaRide's answer would do what you ask, but be aware I totally agree with his closing statement.
In a very general way, I would probably tackle this in more or less the following manner:
In a custom module
Implement hook_cron, this would builds an array containing all emails that match the criteria of your choice.
Loop through the array and send the emails via drupal_mail. Do look into the function
_user_mail_notify() kiamlaluno linked to up above.
If user has never logged in, you may need to send a reset password mail (not sure here), possibly using a combination of drupal_mail and hook_mail_alter to customise either the registration or the reset password email to your taste.
Hope that helps, I know it is a bit sketchy as I have not implemented anything similar in my Drupal experience so far, I was just pumping out a few ideas on how one could notify users in a secure manner. There are many alternatives to having a plain-text password in the email.
Good-luck friend, cheers to you & happy new-year!