As you've probably discovered after years of taking digital snapshots, keeping a photo library organized can be a nightmare. Far and away your best ally: tags, which are little descriptors attached to each photo.
Unfortunately, it's a major hassle to manually assign tags, which is why I was delighted to see the new automatic-tagging feature in the just-released Google Picasa 3.5.
I've used this photo-management software for years, but it's never been this adept at organization. When you first run the new version, it starts scanning your library for faces, automatically grouping those that look similar (and with impressive accuracy, based on my initial tests).
To get started with face tagging, click the Scanning option under the new People section in the lefthand toolbar. (Depending on the size of your library, it might take Picasa several hours to complete its initial scan--but you can start tagging while it's working.)
You'll immediately see a batch of faces in the main pane. Click Add a name under any one of them, type the person's name, and then hit Enter.
In the dialog box that appears, click New Person, and then click OK. (You can also supply a nickname and/or e-mail address at this point; Picasa can sync these tags with your Picasa Web Albums.)
Repeat the process with other faces. If you want Picasa to ignore a face (you might not want to tag everybody, after all), just click the little x in the corner.
Each "new person" you add creates a tag in the aforementioned People section. Click one of those tags to see all the matches Picasa has detected. You can refine these matches further by selecting one or more photos, then clicking the green checkmark if they're accurate (i.e. the correct face) or the red x if they're not.
The more you fiddle with this feature, the more sense it will start to make. Keep in mind that all this scanning and tagging makes no actual changes to your photos. Ultimately, it's just a quick way to find all your photos of, say, Fluffy the Dog, or your Uncle Ed. Great stuff.