Dockyard is a "dock switcher" for Mac OS X. It allows you to have multiple docks and switch between them with one click. You can even switch Dashboard configurations along with your dock. But Dockyard Manager is so much more. Unlike the menu extra and widget versions of Dockyard, Dockyard Manager is a fully-fledged dock editor. Not only can you create, rename, reorder, and delete docks, but you can also add, remove, and reorder the individual items in them. Finally, you can change docks straight from the current Dock by accessing Dockyard Manager's dock menu. Although most of Dockyard's bugs are gone, it is still recommended that you back up your Dock before using it. If your Dock won't switch, try logging out and logging back in.
Note that Dockyard Manager is INCOMPATIBLE with the Dockyard menu extra prior to menu extra version 1.6 (although your docks will be saved). If you want to keep using the Dockyard menu extra, you MUST upgrade it to version 1.6 before installing Dockyard Manager.