Till now, I've been playing around with different Javascript frameworks. I loved Scriptaculous in combination with Prototype. But a few days ago I came across JQuery, a lightweight framework that also seemed to be interesting and after some Google searches I found out there are plenty of "lightweight" Javascript frameworks I didn't know yet...

My question, what framework do you love most (or combination of frameworks) and why?
Do you combine different frameworks?
What are you using such a framework for?