Is there any way to make Flickr Pro photos "unlisted" to random passers by, but let anyone with a link to the Collection access the photos without having to login?
I understand that's not really secure, but that's fine, this is the level of security that fits my needs – giving out the link is similar to handing someone a print: you never know where it will end up.
Best Answer
Yes you can do that using the 'Guest Pass' feature :
Open your set, click 'Share This' and generate a Guest Pass (a unique URL that you can send by mail ...)
Here is the Flickr Faq about that (http://www.flickr.com/help/with/privacy/):
*You can share public photos in your photostream by copying the URL in your browser's address bar and pasting it an email. Anyone can see your public photos anytime, whether they're a Flickr member or not.
But! If you want to share private photos with people, use a Guest Pass. You can grant anyone access to the entire contents of one of your sets, including any private photos it contains. A Guest Pass is actually just that URL. This means that whoever sees that URL can access the set and all the photos inside it.
Go to the set or photo you want to share and click the "Share this" button. Then we'll help you either send an email to friends via Flickr or make a special web address (URL) that you can share.
Or if you want to give someone a link to see everything in your photostream including friends and family photos...*