I like YUI a lot - especially the components they've written for it. The YUI Rich Text Editor blows away everything else I've tried. And on a current project,

I like YUI a lot - especially the components they've written for it. The YUI Rich Text Editor blows away everything else I've tried. And on a current project, I'm using the YUI DataTable to get in browser sorting (and it does a lot more than that, too). But for day-to-day stuff we use Prototype or jQuery because it's easier to write.