Share 'What Javascript Framework do you use?'

In the old days, people hacked their own javascript or downloaded disparate scripts from one scripting site or another. Today it's much more efficient and elegant to code a site with Web 2.0 features using a Javascript library or framework. Which one do you use? And why do you like it? Mootools JQuery Prototype / Script.aculo.us Dojo Ext JS Yahoo! UI Spry I use Mootools for almost all my Jav…