Panorama Point
Originally uploaded by signal11.

Alright if you view the photo again, you’ll see that I’ve added 2 notes to it. How do you add notes to a photo you might ask? The most obvious way: you highlight the region, type your note and click save. What does Flickr do? Exactly that.
When you view the photo and put your mouse over the photo, the regions I marked with notes will appear. When you mouse over the region, the note appears.

Update: I see how they did it. They used Flash to allow notes be added to photos. I thought they had used Javascript Ala Google Maps. Not nearly as cool but still useful nonetheless.