Keyword Assistant is a stellar iPhoto plugin that dramatically simplifies the chore of adding keywords to images. In fact, one could go so far as to say Keyword Assistant brings tagging to iPhoto, but we'll let you judge for yourself. As you can see, it adds a menu item to iPhoto, as well as a simple, smart popup window that allows for comma-separated tags. We call this keyword entry utility 'smart' because it can auto-fill
This can't-iPhoto-without-it plugin just went Universal, and it is still provided as freeware from Ken Ferry's .Mac homepage. KA requires 4.0.3 or 4.0.4 and Mac OS X 10.4.7, and is localized for English, Danish, Dutch, French, German, Italian, Spanish and Swedish.