A couple of days ago I was in Rome with my girlfriend, Elina. Typical tourists as we were, we kept shooting photos with our digital camera. Storage is no longer an issue, I had my 20GB iPod photo with the camera conector. Most of the photos were shot in crowded places, some of them nearly packed with other tourists shooting their own photos and video.
"Have you ever considered how many photos around the world are you in?" Elina said.
I guess a lot, but there is no way to find them, is there?
Then I realized that had I bought the portable GPS receiver I wanted to (in order to geotag  my photos), me and others could search for on-line photos with the GPS coordinates and date of my journey. Something like "search for photos taken at the Coloseum on 2005-11-21 between 13:00 and 13:30". If people geo-tagged their photos, I'm sure I could play my personal version of "Where's Waldo" with some success...
I do not think that this could be done now, I do not know of a service that would allow me to query using GPS coordinates and date (or is there?), but wouldn't it be realy cool if Flickr, Google, or Yahoo! supported such a feature?
BTW, you can find a photo of me here. If you were in Rome during the last weekend and you think I'm in one of your photos, it would be fun to send it to me!
 Read How-to: easily geotag your Flickr photos, HOW TO GPS Tag Photos: Flickr, Mappr, Google Earth....