Download it here.
Why is it useful? If you've got multiple Salesforce.com credentials to keep track of, this extension allows you to keep them organized. Once you've entered them, it only takes two clicks to log into any instance in either a new tab or a new window. Working on several different projects through out the day, I'm frequently in and out of production and sandbox orgs. With this extension, I'm typing my credentials a lot less frequently.
Before you install it, take a look at the author's security note to decide if this extension will work for you in your working environment. If not, there are plenty of non-Salesforce specific credential managers out there.
++++++++++++++++++++++++++++ Security Notice (2012/3/7) ++++++++++++++++++++++++++++ This extension throws Force.com login username and password via POST when logging in with new tab, and via GET when logging in with new window. Throwing information via GET means there will be URL containing username and password shown in the browser history. For those of you who do not want your username and password shown in your browser history, please do not use 'window' button to login with new window. Please also be aware that if anybody has access to your chrome browser, he/she can look into localStorage where all your force.com credentials are stored in. Your credentials are not encrypted since anybody can look at the source code of the extension and somehow decrypt it. ++++++++++++++++++++++++++++
