Picasa Web Albums gets new link "unlisted gallery" at the top of the page that lets you share public and unlisted albums. The URL includes an authentication key that can be reset from the settings page.
"Albums set to the most private visibility setting - 'Sign-in required to view' - will not be displayed unless the person viewing is included on an individual album's 'Shared with' list. Your unlisted gallery has an authorization key in the web address; this key is a combination of letters and numbers which makes the web address very difficult to guess," explains Google.
Have you ever wondered why Gmail's address includes a parameter called "shva". According to Mike Sego, a former Gmail engineer, "shva" is an acronym for "should have valid authentication". Apparently, the parameter is only included after a successful authentication.