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.
The debut version, awarded "Community Choice" for its ease of use.
If you need high-end rendering integrated directly into your workflow, SketchUp Studio is the only version that tackles the common "poor rendering" criticism often seen in user reviews.