I recently ran across a drop in html WYSIWYG editor that works well with Geeklog. It is free and distributed by interactivetools.com. It only works with Windows IE 5.5 or better, but degrades to the normal textarea with other browsers. You can see it in action here by submitting a story or comment.
It is a full featured editor controlling formating, fonts, colors, images, and tables. It even has a full screen option.
It is easy to install only requiring a simple addition to a few theme templates. I have put together a modified package (modified to work with some Geeklog themes) with installation instructions. You can get it from the downloads section or directly here.
Enjoy
TomW