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)?