al3x.de

User/File Management, downloadable files are session bound and .htaccess protected against hot-linking. 
Users are not wp users. Files are displayed in a jQuery powered filetree. 
Now with public download area, public downloads are not session bound. 
You can even tell wp which subdirectory of the public folder should be used as root-directory.

al3x_file_manager

A neat User/File Management with a jQuery frontend to display customizable directory/file tree. Now with public download area.

KaeruCT

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.
The query string parameter that should contain the password is <code>pw</code>.
For example, if the URL of your post is <code>http://myblog.com/password-protected-page/</code> and the password is <code>PASSWORD</code>, 
then just append <code>?pw=PASSWORD</code> to it.
If the URL already contains a query string (for example, <code>http://myblog.com/?p=5</code>), then be sure to append <code>&pw=PASSWORD</code> instead.

Password Passthrough

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.

al3x_file_manager vs Password Passthrough