This script by Jesse Ruderman, AutoLoginJ user script, allows you to automatically login to sites that allow you to save the username and password. He is mentioning another similar script, AutoLogin, and an extension, named AutoLogin as well.
Both scripts are based on GreaseMonkey, an extension which lets you to add scripts to any web page to change its behavior. Neither the scripts or the extension seem to be able to deal with HTTP basic authenticati0n, that would be a nice feature for intranets.
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