Apple's home multimedia suite iLife was recently upgraded, while all apps (iMovie, iWeb, iDVD, iPhoto and Garageband) got an upgrade iPhoto got the best and most significant new features. The most useful of these is Faces.
Faces
For a long time now you have been able to tag people on social networking sites such as Facebook linking them to their user profile. Facebook has the ability to detect faces however cannot match them meaning you have to tag every person in every photo. iPhoto takes this to the next level, after first scanning your entire library to detect faces (a job which took around 90mins on my 8418 photos on my 2.4ghz C2D Intel iMac) you have the ability to start tagging people.

Above: iPhoto scanning my library looking for sexy people
Once you have tagged a person in a couple of photos iPhoto will start to suggest similar photos, at first some of the suggestions are wild guesses but by simply clicking a photo once to accept or double clicking a photo to deny makes it very quick and easy to work through your library.

Above: Single click to accept a picture, double click to deny.
The majority of my photos contain people, often from nights out in fancy dress. iPhoto struggled with some of these photo at first and I found myself having to tag photos so that iPhoto could detect the rest.
Above: After a couple hours of tagging I now had a selection of Pete photos. I had to tag myself on numerous occassions, iPhoto could not automatically detect that baby Pete (top right) was the same as sumo Pete (bottom middle).

Above: I now have my three friends tagged in iPhoto.
Tagging photos is all very good but its what you then do with these photos that is interesting. For instance what if I want to see all the pictures of me and a friend or with valentines day approaching all the photos of Laura and myself. I can create a smart album, just as you would in the older versions of iPhoto but this time you have the option of specifying names.
Above: A filter in a smart album.
Above: The filter results of pictures of Laura and myself.
It doesn't end here as iPhoto then lets you print or order items based on those photos producing an instant valentines day present.

Above: Three years worth of presents.
With iPhoto 09 you also have the option for automatically uploading photos to Facebook and Flickr.
Tagging takes several hours especially if you have a large library but its well worth it.