I just installed the latest version of Picasa, which was released yesterday. As a PC guy, I've been a little envious of my Mac friends who use the iPhoto '09 because of a cool feature called Faces, which is essentially a face recognition tool to help with photo organization (i.e. tagging).
If you are like me and have thousands, if not tens of thousands, of photos laying around on various local drives and on the network and you're having a hard time organizing those photos, the Faces feature (or People as Picasa calls it) can help you organize your photos that have people in them in a pretty cool and speedy way.
The feature is very simple to use. Once you finish installing Picasa 3.5 (if it's an upgrade), it goes through your entire photo library and finds all the faces in each picture. It then prompts you to identify a few of the faces it found and based on that, it tags other photos that contain similar faces. From there, you have the opportunity to accept or reject Picasa's guess (or suggestion) for each photo individually or en masse for each person. So far, it has been about 90% accurate for me and where it made mistakes, it was for a good reason like where mixed up my two aunts or my cousins.
Picasa 3.5 introduces a couple of additional features such as the Geo-Tagging and integration with Google Maps. I've yet to explore those additional features, but thought it was a good idea to share my excitement about the People tagging first.