RenderPlus is a company staffed by some of the people who used to make Aritek (purchased by Softdesk 12 years ago), and they have a list of add-ons for SketchUp that you can download for free from here:
- RpWallMaker creates wide walls and cut openings.
- RpTreeMaker create custom tree components.
- RpEdits lays out your 3D components and ones from the 3D Warehouse.
- ProjectSketch selects and places commercial office furniture.
The site also has non-free add-ons, such as for rendering, generating BOMs, and editing 3D components.