Here are a few best features of iPhoto '09, including Faces and Places, which recognizes people in your albums and the locations you took those photos in.
Cool stuff: separating your photos by people, which you can then scrub (move your mouse) over and see all the images of them you have. iPhoto is smart enough to try and recognize which people are which with semi-decent accuracy, and you'll have to confirm each one as you go. Once you're done, you have a corkboard full of Polaroids of the people you know.
Geolocation is great for travelers, and if you go to a lot of different countries, your "Places" section will have a rich map made up of all the pins you've been to. You can fill in location data yourself, or if you have an iPhone or other GPS-enabled camera, it's automatically filled in for you.
Other cool stuff is Facebook and Flickr integration (includes tagging as well), plus themed slideshows. [iPhoto]