YUI 2.3 ReleasedYUI 2.3 came out almost 2 weeks ago and I forgot to mention it. Several old feature have been improved and revisited, and 6 new feature have been added. I am most impressed by the color picker (great extension for WYSIWYG editors and theme editors), deferred image loader (allows for low res images, followed by high res versions for faster page loading), and the improved CSS package (more layouts, better normalization and fonts). Anyway, it is better than before and there are a lot more packages to include if you like.
Ultimately, though, there is only one piece that you need on every project and that is yahoo-dom-event.js. This minimized file (36k) contains the yahoo.js, dom.js, and event.js core functionality. Using this file you will have all the DOM manipulation functions that enhance productivity and one of the best event hander packages that exists today. It also, contains CustomEvent, which is my favorite part of the YUI library.
On a side note, I had dinner with Eric Miraglia of Yahoo! about a month ago and asked for some changes to the connection manager. I tend to think of AJAX requests as another event type and would like to see it incorporated with the Event package, or as a CustomEvent. This did not happen, but there has been some improvement to the ConnectionManager allowing the Object to behave more like an Event.