An easy to install library for Javascript and PHP that you can use to create familiar dropdown menus like in desktop applications. It comes with many options, including checkboxes, radios, keyboard navigation, shortcuts, ability to add or delete items on the fly, etc. ZZEE DHTML Menu has a simple API and is fairly easy to install and embed into your existing HTML or PHP pages.
Benefits: Search engine friendly (PHP version only). Compatible with the major browsers. Small file size, little download overhead. Comes with four ready made styles.
License for unlimited websites / intranets / HTAs. The program is free for non-commercial use.