Keyword Assistant lets you quickly add keywords to your photos in iPhoto. You can select multiple photos, press the Keyword Assistant shortcut and add keywords. When you start to type a keyword that you have already used, KA will fill it in for you.
The process goes faster than you'd imagine. I have about 4,000 pictures in iPhoto and I was able to tag them all with keywords while watching just a couple movies. I really wanted this done because now I can use Spotlight to search for ?grandma? and all the pictures of grandma will show up. I can even watch a slideshow of them immediately. It's very neat.
Keyword Assistant is a plugin and is ready whenever iPhoto is open.