Javascript is one of the most important aspects to enhancing web design. It can do so many things, that it can often be mistaken for Flash. Whether you are looking for a news scroll bar, a rotating picture slideshow, or a fancy menu, javascript can be used to do all of that.

Two huge libraries are JQuery and Moo Tools. I have more experience working with JQuery, but Moo Tools seems extremely flexible as well. They both enable you to easily create functions that help enhance the users experience with the website.