Picasa has a facial recognition capability which attempts to match faces in different photos. Given an album of photographs and told to "Add Names", Picasa scans through the whole album or through all the albums looking for things that look like faces and groups the same face (as far as it can tell) together, and shows them to you in groups so you can name them, and identify those that Picasa identified incorrectly.
Please note that at this time, if you cannot identify a person from a picture, neither can Picasa, although it can try to make a guess and provide you with what it thinks are promising candidates. In fact there are situations when even if you can identify a person, Picasa is unable to do so. So, if you expect it to identify any face automatically, you will be disappointed.
Sometimes Picasa identifies as faces things that are not faces - for example, it has presented elbows, chair corners and even lighting effects as faces. In that case, you can tell Picasa that those are not faces, or to skip identifying them, or to ignore them.
It is of course possible to correct mistakes, and if you give two different names to the same person in different photographs, you may merge the pictures together to one identity. It is possible to associate email addresses with each face, and to identify the preferred photo to show for a given face. Finally, it is possible to find all identified instances of a face in the albums so you can find all pictures with a given face.
So, while it will not provide 100% accuracy for identifying faces, I have found that it has picked up on faces that we would normally skip over because they are small, in the background, or generally not the focus of the photograph.