With the free version of Sketchup and the free version of Vue Infinte 6 PLE
Save your Sketchup object as a .skp file.
Open up Vue Infinte 6 PLE, Go to File, Import Object and import it.
Then use the Export Object option nd save as a OBJ.
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Also free is Blender 3d. It can import sketchup models using the script from blenderartists.org. It is a python script and so is somewhere in their python forum.
Blender also has a OBJ exporter.
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Also, it is possible that the current version of Deep Exploration can convert Sketchup files.
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Better still, go read the tutorial I wrote on Truespace. Which while it can not read Sketchup files, is a better modeler. And, Bryce can read Truespace files without having to install a bunch of "in between" files.