WYSIWYG Plugin for WordPress
It uses the Open Source TinyMCE and an image browser that comes with it making it easier to write and edit pages and posts in your weblog.
It works with pretty much any Mozilla based browser such as Firefox (Mac, PC or Linux) and with Internet Explorer for Windows.
Update: I won’t be supporting this plugin anymore. WordPress 2.0 has TinyMCE built in and there is no reason to keep working on this since it is so easy to upgrade.