Custom menus are finished! Yay!
Multi-site is all set.
The look of the WordPress admin has been lightened up a little bit, so you can focus more on your content.
There are a ton of changes, so plugin authors, please test your plugins now, so that if there is a compatibility issue, we can figure it out before the final release.
Plugin and theme *users* are also encouraged to test things out. If you find problems, let your plugin/theme authors know so they can figure out the cause.
Info and download: http://wordpress.org/development/2010/05/wordpress-3-0-release-candidate/