ZZEE DHTML Menu 2.1.0
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.