Originally downloaded March 18, 2007. Even in its 2006 iteration, iPhoto still hasn't made use of keywords as intuitive and quick as it should be. Maybe with this, I can finally convince my wife that she should spend some time tagging our photo collection. 
Keyword Assistant gets very good reviews from most sources, and has been around long enough to be very mature and stable.
Version as tested: 1.9.4.
Update 9/5/12. This software is no longer in development or supported.
