Scriptaculous is one of many great new JavaScript libraries created to answer the call for well written 'Web 2.0' JavaScript libraries. Written by Thomas Fuchs, scriptaculous has many features that can be used in AJAX-ified applications, drag-and-drop effects, and a whole slew of visual effects. The drag-and-drop effects, most notably the sortables, caught my eye because the look great, they are so easy to implement, and they're just so much nicer than the standard listbox with up/down arrows that we see in most of today's applications and administration tools.
moo.fx is a superlightweight, ultratiny, megasmall javascript effects library, written with prototype.js.
It's easy to use, fast, cross-browser, standards compliant, provides controls to modify Height, Width, and Opacity with builtin checks that won't let a user break the effect with multiple crazy clicks. It's also optimized to make you write the lesser code possible.