Tagcow is one of the best image tagging services. It offers both descriptive tagging and people tagging for all your images.
Descriptive tagging identifies objects within your images and turns them into searchable meta data. For even more fun you can use people tagging to identify the people in your photos. Find that silly photo of your friend Tom sleeping at his desk easier than you can find directions to your job interview with Google Maps. Tagcow is also laying the groundwork for a simple image management system for the everyday shutterbug. Their Flickr synch feature is just the tip of the iceberg in what’s to come. And finally, it’s free. So you have nothing to lose. 
