Any digital camera worth its salt will magically embed some contextual data into an unseen portion of the image file. This information usually includes the date/time when the picture was taken (or "January 1, 1970" if you never got around to setting the clock on your camera), whether the flash was used, possibly even the location where the photo was taken (if the camera has GPS, or the photographer uses one of a few special geotagging iPhone Apps).

Once the photo is posted online, we can use Facebook to tag people in the photos. We can add tags, titles, and descriptions.

What other data can we associate with a photo, preferably automatically (because I'm a lazy photographer)?

If we know what time the photo was taken, and we know where the photo was taken, it would be quite simple add a “Day” tag if the photo was taken between sunrise and sunset, or a “Night” tag otherwise.

Weather

Was it snowing when this picture was taken? Was it the hottest day of the year? Was visibility less than a mile?
I'd love to be able to search for photos of hot days in Minneapolis, or clear sunny photos of the Golden Gate Bridge.

This information could come from one of many different sources.
Weather.gov has a ton of information about weather at airports all over the world, and there are plenty of ways to access this information. Alternatively, the camera could have some basic climate sensors included, or connect to something external like the Node Clima to record this information at precisely the same time and location the photo was taken.

Tripod Info

What if my tripod had an RFID or NFC identifier tag, so the camera could tell if a tripod was used to take a picture, and record some information about that specific tripod?
This method could also detect whether the camera was inside an underwater housing.