Picasa 3.5 scans all the photos in your collection, identifies the ones with faces, and groups photos with similar faces together. It’s easy to add name tags to dozens of photos at once by clicking “Add a name” below a photo and typing the person’s name. Once you’ve tagged some pictures, you can make a face collage with one click, easily find all your pictures with the same two people in them…
Picasa 3.5 is a free download for Windows or Mac.