Sketchup - New Version and Conversion.
Charlie Judge
Posts: 13,247
3D Warehouse has a large number of free 3D models but they are all in Sketchup (skp) and conversion is sometimes a problem However there are various free plugins or converters to export an obj file
The new version (2016) of the free Sketchup Make is now available and installing it will give a free 30 day trial of Sketchup Pro which will allow you to export obj files for 30 days. This free trial seems to work even if you have had previous versions with free trials. Just unistall the old version fist. While this seems to be the best converter, once the free trial is over you can go to 3D extension warhouse and install the free Lipid obj export plugin directly into SketchUp and/or you can downloed the TIG obj Exporter as an rbz file from SketchUcation and then install the file in Sketchup.
Also Babel3D and Greentoken are online converters which can be used to convert the skp file to obj.

Comments
TIG exporter is the one I use and it seems to do a more than adequate job.
And no matter which exporter you do use, often cleanup is required...even if it is just redoing the textures.
I used to use some Sketchup models but found them to be too low poly and most of the textures very low quality. I use them if I have to for background stuff, although I haven't in awhile.
Quite a few of the furniture props are of reasonable quality...
Some of the other houseware items are too.
been my experience that sketchup models from the warehouse are more of a pain to cleanup and not worth the trouble since they are designed with sketchup and that specific use in mind. As much as users around here have issues with just switching between 3delight and iray shaders, most would really be putoff with the work invloved in getting a decent sketchup model working in DS.
I do like some of the furniture...and most of what I've done I spent more time rigging them than actual cleanup, so I think it's a fair deal, because if I didn't rig them, they'd work just fine as solid prop pieces...minimal texture work, etc. Just tweaking the surface settings, but I do that with just about everything, anyway.
It's the vehicles and buildings that really drive one close (or over the edge). I have much better luck with ones from Blendswap...especially buildings (there's a really sweet house over there).