There are plenty of drop-down menus out there, but this one not only works well, but it’s easy to use, and has a “reuse-friendly license” (you can use it as long as you leave in the copyright notice).   This one is from SCL Software.   Unfortunately, some of the website is in Spanish, but I kinda was able to figure it out anyway.

The drop-down menu code is available here.  I originally found it in the daily email from (a wonderful resource), on this page.