First and second thoughts, jQuery, YUI and charting
The YUI, the Yahoo User Interface library has a different feel. It's more structured and has very well thought out utility abstractions and copious amounts of documentation. The data and connection utilities are very nice. It comes with a set of widgets and styles but they feel more like a building material compared to the prefab construction of JQuery UI. It is modular but much larger than JQuery, though it also does much more. Considerably more code is necessary to get things working and some widgets have annoying interactions with my existing styles and layouts. The documentation doesn't show the same method for working with a control twice and the demo coding style is inconsistent in places. The flash-based charting library is very good and works as expected on IE6 without resorting to a canvas plugin but lacks zooming and is considered “beta”. All told, I spent three full days learning the APIs and porting the existing demo, one of those just reworking my existing layout and css to fit the YUI grid model. The payoff is that I improved my code by adopting some of the Yahoo style and they've evidently put serious effort into cross-browser consistency.