Creating a one-time password, and send the email
Login One Time comes closest to your requirements. From its project page:

Login one time provides the ability to email one-time login links to users.
Features
Rules:

[user:login-one-time] token on user events.

[comment:login-one-time] token for node author on comment event.

User profile - &quot;Send login one time link&quot; button.

VBO - Create a view and send login links to the view. Checkout View
Reference for ideas on how to embed views.

Block - Select a user and send the link. You can use this block in
nodes as a CCK field using Block Reference.

PHP - Print a button in a template or directly send the emails using
the functions described in the README.