Thank you very much for the links. There is a lot of interesting information about the plugins, but is there not ONE page for downloading these plugins? I found it very exhausting to search these sites to found (hopefully) a download link.
It depends on the plugin you want to install.
Many plugins are part of the contrib-plugin package (on linux) or the installer on windows (and of course part of the nightiles).
But there are also many interesting plugins, that are not (yet) included into the contrib-plugins package.
These plugins have to be downloaded seperately.