Most people have thousands of digital photos in their collections. One of the cool new features available in Picasa's Web Albums (which kind of got swept away in the torrent of Chrome coverage the last couple of days) is the ability to tag faces in your photos quickly and easily.
If you have photos on a Picasa web album already, you can enable the feature in settings. It then scans all your photos in the Picasa web album, before grouping similar faces together. You then work through those by giving name tags.
You'll quite often have to repeat the same person in different groups of photos, so it's a far from perfect solution. But it does make the tagging process much quicker than manually going through every photo.
It's also secure, enabling better searching and more freedom for specific collections of photos, and you can share tagged photos with the people in them easily as well. Any images you share via Picasa also only shares the nicknamed tag you've put on the photo, not any further details like contact information.
This is a really useful development for cataloguing your photos, but why it's only available online and not in desktop photo management software like iPhoto or Picasa for Windows is beyond me. Hopefully we'll see it rolled out to desktop applications sooner rather than later.