For as long as I can remember, Ken Ferry's Keyword Assistant has been the only iPhoto plugin (at least, AFAIK) for adding some real power and, dare I say, "tagging" to iPhoto's keyword system. Now there's a new kid on the block by the name of Keyword Manager, and it definitely brings some interesting features to the table. Keyword Manager uses two separate windows that both have their own keyboard shortcuts for toggling visibility. The first is a Quick Assigner window (pictured on right) which, as you might guess, allows you to quickly
tag add keywords to your photos. It offers keyword completion (via a slick drop down menu) and support for multiple word keywords. The second panel (on left) is a very nice Keyword Manager, allowing you to organize keywords in a hierarchy to help keep your organization in check. Naturally, since both of these panels have their own keyboard shortcuts, you can call them even when in iPhoto's full-screen mode. All this is done with a very nice iPhoto Adjustment Panel-like UI.
Now Keyword Manager is pretty impressive, but I think they're reaching just a little too high by charging $19 for a plugin, especially in light of Keyword Assistant being free. I think $14.99 would hit the sweet spot for a plugin like this, but then again, I admittedly haven't plunged into organizing my increasingly sprawling iPhoto library yet, so YMMV.
[Update: Commenters Jools tipped us off to promotional codes at VersionTracker that drop the price to $12 in the US or €12 in the EU: VT-542U845 (US) and VT-542E845 (EU). No word on how long those codes are good for, but that's a much more reasonable price in my book.]
Keyword Manager is available from Bullstorm, and a demo is available which allows you to assign keywords 75 times before requiring a license.