An appealing alternative to standard hard to navigate dropdown menus; options are shown in one large panel that allows for categorization, icons, etc. Article at Sitepoint: http://www.sitepoint.com/blogs/2009/03/30/mega-drop-down-menus/
Examples:
- http://www.actionenvelope.com/
- http://www.foodnetwork.com/
- http://www.emc.com/support-training/index.htm
I suspect it would be a challenge to implement in Drupal, but not impossible.
BioMed 2.0 
Nice! (this interface is pretty bad, since it doesn’t really say ‘add comment’ 😦 )
Another example from the Gates Foundation site http://www.gatesfoundation.org/Pages/home.aspx — interesting also that there are 2 separate menus in the main nav bar.
Another example of mega menus here at UCSF, on the site for the Children’s Hospital: http://www.ucsfchildrenshospital.org/
Adaptive Path worked on the design. Makes our Virtual Home colors look pretty tame!