Yahoo has released a pretty impressive collection of JavaScript libraries on their Developer Network. The new libraries are collection they're calling Yahoo! User Interface Library. The new UI library consists of about everything you need to build modern web applications—AJAX (Connection Manager), animation effects, drag-n-drop effects, event handlers and basic DOM umbrella for miscellaneous DHTML handling.
They've even released a few nifty UI controls (widgets) that seem pretty feature rich. These include a Calendar, Slider and a TreeView control. The slider control seems very solid and I like the "task" mode of the TreeView control (it allows you to place checkmarks next to the tree items—which would be useful in removing nodes from a tree.)
I'd also recommend checking out the new Yahoo! User Interface Blog, which is designed to talk about changes and additions to the UI Library. They've also published a Design Pattern Library which talks about common design problems and provides some solutions. They Design Pattern Library has an interesting entry on "Auto Complete", but they have not yet posted any actual code.