XShelf enhances drag and drop in MacOS X by letting you "pause" drag and drop operations, as well as have multiple drag and drop operations in flight at once.
You can drag files or folders from Finder into the shelf and they will sit there until you drag them out. Once you drag them out, the drag operation will finish as if XShelf were never involved. You may drag individual or multiple files or folders, as well as text clippings and URLs to XShelf. With XShelf, you no longer have to shuffle windows so that both the source and target of a drag and drop are onscreen at the same time.... More Info »
It also has support for Services, better integration with the clipboard, and a commandline utility (XShelver) to add items to XShelf.
In many ways, it is a replacement for the old NeXT shelf (and the Newton shelf, but never having used the Newton, I have to take this on faith :)