Janis Elsts

Normally when you login to your blog WordPress will create a session cookie that is used to authenticate you. If someone was to steal the cookie they would be able to use it to get full access to your blog without having to know your password. This plugin prevents that from happening – it makes the cookie specific to your IP address, so it won’t be usable from a different computer.
Use this plugin if you have a static IP. If you have a dynamic IP address and it changes often you will get logged out frequently.
A bit more info can be obtained from the <a href="http://w-shadow.com/blog/2008/07/12/safer-cookies-plugin-for-wordpress/" title="Safer Cookies homepage" rel="nofollow ugc">Safer Cookies homepage</a>

Safer Cookies

Ties the WP session cookie to your IP address so that it can't be used to get access to you blog from another computer.

atdev

The phpMyDirectory WordPress plugin allows wordpress users to log into phpMyDirectory. The plugin is configured in the WordPress admin area.
Users only need to log in once and they will be automatically logged into phpMyDirectory. This allows phpMyDirectory to be used alongside WordPress and for a directory to be easily created for WordPress.

phpMyDirectory

Allows wordpress users to automatically log into phpMyDirectory.  The sessions are shared and accounts are created automatically if they do not exist.

Safer Cookies vs phpMyDirectory