This page lists all the important ones.
You should probably visit the first 3-4 in the list, those have the most content:
, I have checked all sites - very useful, thanks a lot - but i have problem using the objects. Could you please explain a bit about how to download them? I chose download for archicad, downloaded file, unzipped.... but I must be missing something else. I've never downloaded objects before, so just have no idea how to deal with all these different format files. Thank you!