Components
Collection of assorted stuff...
SketchUp Components In the 3D Warehouse, a collection of objects to help you populate your 3D models. These models used to be downloadable, in mass, as a bonus pack(s) from the SketchUp website. Instructions on saving Warehouse search as a local collection through the Component browser within SketchUp.
2D
Entourage Arts Non photorealistic entourage elements
RpTreeMaker Fractal tree component maker - Free - Windows
2D, 3D and Styles
3D
3D Warehouse SKP - Free
3D Lapidary Gem cuts DXF
Andersen Windows A stand-alone applications which generates dynamic 3D SKP models of Andersen Windows - Free - Windows
BYTESYZE STUDIO SKP models packs for sale
BIM object (formerly Autodesk SEEK) Models in many different file formats, including SKP
DesignConnected Furniture, lighting, and accessories - multiple file-formats - Free, Fee
Dynascape 3D landscape design content.
engineering models
PARTcommunity CADenas models in the 3D Warehouse (+50,000).
SimLab (paid) plugins will also let you import SolidWorks or IGES formats meaning you could grab CAD data from websites like McMaster Carr to import into SketchUp. Other SketchUp importer plugins.
TraceParts Main website to exports models useful to engineers in assorted formats, including COLLADA.
TraceParts Models in the 3D Warehouse (+11,000).
Enkeboll Designs 2D and 3D millwork line art in DWG/DXF, JPG, EPS
Free AutoCAD blocks Traditional 2D CAD symbols
Gandoza.net 3D models in several formats - Fee
Great Buildings Famous buildings - 3DMF > DXF via DesignWorkshop - Free - Windows, Mac Classic
LOWPOLYGON3D Some Free, mostly Fee
Molding Profiles SketchUcation thread with wood molding profiles
Model 3D CAD symbols, 3ds, some free, mostly fee
Osborne Wood Products CAD files of their wood products.
Simplified Building Concepts has SKP models of galvanized Kee Klamp pipe components and aluminum Kee Lite components. Click on the details of each element.
SketchUp Engineering Toolbox Plumbing (flanges, elbows, bends) - Angle & channel iron, etc. (SketchUp plugin available - See description at SketchUp Blog)
Sketch-Shop authorized reseller
Solopack 3D garden elements solos-art.com - Free, Fee
SteelSelect SKP
Sweets Network Construction network with 3DWarehouse building product models
The SketchUp Component Collection Components for custom-built PCs.
Tomsdesk 2.5D vegetation, trees 2.5d trees from sets A & B 2.5d trees from sets C & D - Free
Toucan Virtual Museum Fish, flowers, and insects. 3DS and BMP