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.

brechtryckaert

Limiting the cookie expire time of password protected posts. By default the cookie set when entering a password to access a password-protected post will be active for 10 days. This plugin turns this cookie into a session-based cookie. This means your visitor will need to re-enter the password if the browser was closed prior to revisiting the post.

Instant Cookie Expire

This plugin turns the cookie for a password-protected post into a session-based cookie.

Safer Cookies vs Instant Cookie Expire