If you don't know what jQuery is:
jQuery is a lightweight JavaScript library that emphasizes interaction between JavaScript and HTML - Wikipedia
For me, it is another JavaScript library that you don't want to miss. It is a single JavaScript file, containing all the common DOM, Event, Effects, and Ajax functions. Just add this the <head></head> tag:
Not only that, you can add in your own methods and functionality in a mechanism called plugin and put it just below the jQuery.js like this:
The community behind it is quite a number. This is evidently seen in the plugin submitted by the user in the plugin section. It cover the most AJAX functionality that you can think off. With all these, I'm certain that it can help your UI design alot. Maybe I shall implement it in my future Blogger Template design.