We want to do everything we can to help people extend SketchUp's functionality and make it even more compatible with other software. If you need information on creating a plugin for SketchUp, or if you want to read or write SketchUp files, this page is the place to start.
The easiest and most powerful way to write plugins for SketchUp is to use SketchUp's Ruby API. You can use this API to write your own tools, as well as query and influence SketchUp models. The scripts you create are freely distributable, and the will work in both the free and Pro versions of SketchUp. The SketchUp Ruby API is free to use.
If you're interested in using the SketchUp Software Development Kit (SDK) to read and write SketchUp files, or in writing a custom exporter that works from within SketchUp Pro, you need the SketchUp SDK. The SDK contains: