I wonder how many of those are just mirrors of Linux distros and other open-source software, how many have more interesting things (including software), and how many of those were deliberately configured to be open for sharing. There is the somewhat-well-known filesearch.ru if you want to look for things on this non-HTTP part of the Internet. (If I remember correctly, Google used to index FTPs too and you'd get plenty of results with the right queries, but that seems to have mostly and silently disappeared...)