Thomas Fuchs has released a Drag-and-Drop extension for Prototype.

Generic drag-and-drop

• Any element can be declared draggable and/or a drop target
• Callbacks (onDrop, onHover)
• Integrated CSS class change to add class drophover if hovering over a possible drop-target
• Constraints (drag only vertically or horizontally)
• Uses the Effect2 library for transparency and snap-back visual effects

Sortable lists (UL, OL)

• Sortable float containers (DIV containing any sort of floating elements)
• Callbacks for sortables (onChange, onUpdate)
• Easy to do sortable lists that also can drag-and-drop to another list
• Sort order can be serialized to use with Prototype’s Ajax functions

Works with

• Safari 2.0; Firefox 1.03, 1.04, DP 1.1; Internet Explorer 6.0; Opera 8.

View simple Draggables and Droppables for Prototype, and sortable floats and lists

The code to do this work is made pretty simple:

Sortable.create('floating-images', {tag: 'div', overlap: 'horizontal', constraint: false});
	
Sortable.create('firstlist',{containment:['firstlist','secondlist'],
    onUpdate:function(element){alert('The first list was affected during the last drag.')}});
	
Sortable.create('secondlist',{containment:['firstlist','secondlist'],
    onUpdate:function(element){alert('The second list was affected during the last drag.')}});

And you can allow serialization of the list:

<ul id=\"mylist\">
   <li id=\"mylist_1\">asldjkflk</li>
   <li id=\"mylist_2\">weoiowe</li>
</ul>
	
Sortable.serialize('mylist',{tag:'li',name:'mylist'});