The original release. It ran on Windows only. The revolutionary feature was the tool, allowing users to extrude flat surfaces into 3D volumes instantly. It did not have layers or texturing capabilities in the way we know today.
Introduced LayOut , allowing users to turn 3D models into 2D professional documents. sketchup version list
To fully understand the version list, you must know the current Trimble tiers: The original release
Google acquired SketchUp in March 2006 to build a "Geographic Information System" (GIS) for everyone. They released a free version (SketchUp Free) to populate Google Earth with 3D buildings. sketchup version list