Autodesk Exchange Apps" store. This online "AppStore" is integrated into AutoCAD 2013 (and most industry versions of AutoCAD), Inventor 2013, Alias 2013, Inventor Publisher 2013, Revit 2013, Simulation 2013, Vault 2013 and Autodesk PLM 360.
Exchange Apps contains hundreds of free- and shareware add-on applications and block libraries by third-party vendors (Autodesk developers) and also by Autodesk. All applications were tested by Autodesk and are very easy to install - expect similar user experience as in Apple AppStore or Google Market (Play).
One of the interesting AutoCAD plug-ins available in this Apps store is the "SketchUp Import" (developed by Autodesk). This easy to use tool converts SKP files from Google SketchUp into 3D models in AutoCAD. Model geometry is composed from block references containing 3D meshes. An example of a converted 3D model (battery):
Exchange Apps for AutoCAD, Inventor and Revit contains also several applications by CAD Studio.
Plugins developed directly by Autodesk bring a new aspect into the structure of the respective CAD product. E.g. AutoCAD now contains 3 levels of functions and modules. These 3 levels differ in the terms of license, quality assurance, support and author's responsibility: internal core functions (with full support and reliability), Express Tools (unsupported legacy add-on tools) and now Autodesk's apps in the AppStore. Autodesk here acts more like an independent vendor, despite adding new functions to its own software product.
In any case - try yourself the Exchange Apps and discover interesting add-on tools - from games, over productivity tools, to full-featured professional applications.