I love the "BugMeNot" extension.
BugMeNot is a site which has a database of sign on names/email addys and passwords for anyone to use when someone does not want to register for a certain site, i.e. nytimes.com. When you install the extension, it allows you to right click in the sign on box and when you click on BugMeNot, it will automatically search for a username and password in its database to enter into the box. Pretty nifty.