Drop Down Menus

Horizontal Drop Down Menus from A List Apart
Article on creating horizontal drop down menus.

Dynamic Drive
Lots of scripts for menus and navigation using CSS and DHTML, from Dynamic Drive, a specialist DHTML site.

OpenCube Site Navigation Software
Their drop down menus look good and can be created using a visual designer.

Sothink
Have a number of tools for creating DHTML menus. Some can be used as a Dreamweaver extension.

Xtreeme DHTML Menu Studio
Has plugins for popular web design software.

Likno AllWebMenus
Javascript / DHTML Menu Maker