Smashing Magazine posted this article on some wonderful ways to use AJAX on your website. I’m not saying there’s a whole slew of new, innovative techniques there, but it’s really nice to have such a large collection of them in one place, with working examples.
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.