Something I stumbled across recently, Kendo UI!
Has a very feature rich set of widgets, and seems quite complete compared to jQuery UI, as it includes some more complete items (Datasource, Templates, Validation to name a few) that you would otherwise have to bolt on as plugins using jQueryUI.
Don’t get me wrong, I’m not bashing jQuery UI, far from it (I still use it as my main development for ui components) but it gives you a pretty good starting point and I’m tempted to use it for a project to see how well it stands up.
It also includes a pretty damn nice looking dataviz component for graphs.
Slight caveat: It’s not free (at least not commercially), but has a pretty good price point for what it offers.