Joomla! Autologin extension allows to auto login a user by only passing username and password thru request variables (user and passw)
You can use it on email, external sites, webapp, ecc....
Joomla Article Preview extension allows to preview an article before publishing, just select your favorite item menù for real article preview in plugin settings.
Create an article, set it to a temporary category and save it. It will appear a new Preview Button in the toolbar.
If all is ok set article to its right category and save.