I've finally rolled out the ability for you to tag other people on the site and objects in photos in your gallery.
Just view the file in your gallery and click the "Tag Photo" link right below the rotate buttons in the Information section. From there on out, click on a person or object in the photo. A box will pop up, allowing you to choose from a friend of yours, search through everyone on the site, or type in a random tag like "ski" or "hood", etc.

Adding A Tag

Tagged Photo
Clicking the check box next to a person tags them and they are notified. You can hover over the captions of tagged individuals and see a floating box around the portion of the picture that they are in. This is great for identifying multiple people in the same picture. A list of photos you are tagged in can be found via your profile.
Try it out and let me know if it is working properly for you!