Yes, if you register as a guest on multiple sites with the same email address, you only need to log in once then you can navigate to other sites. So you only need one password associated with your email address.
On the right side of the site, above the Photos section is a section called Jump to Website. Click in the box marked Choose a site. A list of other websites will appear where you are a guest. Select the site you want to visit next and it will take you directly there since you are already logged into the system.