A word from a sponsor:
Do you prefer to use CSS-like selectors to determine the elements you wish to manipulate?
Do you prefer preset visual effects and interaction flow?
Do you like to depend on plug-ins written by others?
Go for jQuery.
Go for Prototype.js.
Do you prefer a ready-made set of graphical user controls?
Go for YUI.js.
Do you wish nearly complete control over what happens?
Do you fancy a small, atomic footprint on memory usage?
Go with zjrJS.
There you have it.