There is also the Cleaner module.
Nancy Injustice anywhere is a threat to justice everywhere. -- Dr. Martin L. King, Jr.
From: sivaji j.g
In settings.php, you can define how long a cookie is good for.
This is fine but when the PHP garbage collection is not in place or misconfigured it won't work as expected. Well there is a module for that http://drupal.org/project/session_expire