DHTML and Ajax utilities

I found two nice Ajax utilities that can help building web gui.
Both of them are compatible with Firefiox, IE and other browsers.

The first, YUI Library(Yahoo UI) - http://developer.yahoo.com/yui
Is well explained and the API is easy to use.
I didn't use it because I've already built most of the controls like Tree, autocomplete, context menus, etc.
But there are some nice generic objects like drag and drop that can be used in objects we do.

The second utility, Prototype - http://prototype.conio.net
I've found nice functions that can help us to reduce code like the $() function, instead of writing document.getElementById().
Also has some objects that implements HTTPRequest. Nice to check it out, also to learn hot it works.