And now, this thing has hit version 5.9, with full support for Leopard (an entire week early!), and a host of other updates. Actually I'm not sure how much a "host" is, but there's a lot-- a new look, icon reflections (optional, in case you don't want things too shiny), new themes, live previews, and the ability to insert and rearrange items in the Dock using just the eponymous dragging action. Very slick.
DragThing is available over on the website-- a single license is $29, and you can get five and ten packs of licenses for $99 and $149. Trust me-- it's a real drag.