Extensions, also called plugins, extend SketchUp's functionality and can be found in many places on the Web.
Where to find extensions online
The following websites are the best places to download extensions:
Extensions Warehouse at http://extensions.sketchup.com/ is the official extensions site. It is also available directly inside SketchUp and can be found by navigating to Window | Extensions Warehouse.
SketchUcation Plugin Store at http://sketchucation.com/resources/plugin-store-download. This is an extension for downloading and managing extensions, having the largest collection of SketchUp plugins online, with instant install, plugin sorting, and more useful features.