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.

tronhammer

This plugin will mimic a keep-alive ping by utilizing jQuery to perform an ajax request to the current page, which will update 
the session token tied to PilotPress and or any other system attached using PHP built in sessions. Session Slap will also provide 
a settings interface to allow admins to configure details relating to interval amount (which is defauled to 5 minutes less than the 
session.maxlifetime set in php.ini).
A few notes about the sections above:
<ul>
<li>Configuration options are available in the Settings -> Session Slap menu.</li>
<li>Updates built in PHP sessions only.</li>
<li>Attempts to access session.maxlifetime ini settings, but will default to 24 minutes if not available.</li>
</ul>

Session Slap

Session Slap mimics a keepalive ping by making an AJAX request to the current page to refresh active session timestamp.

Safer Cookies vs Session Slap