I didn’t know about this feature either. Flickr Guest Pass:
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 you want to share and click the “Share this set” 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.
You can then expire the pass whenever you want to. I had no idea until I saw it on Daring Fireball tonight. I could see myself using it.